Mutations in the BAF-Complex Subunit DPF2 Are Associated with Coffin-Siris Syndrome.

Variants affecting the function of different subunits of the BAF chromatin-remodelling complex lead to various neurodevelopmental syndromes, including Coffin-Siris syndrome. Furthermore, variants in proteins containing PHD fingers, motifs recognizing specific histone tail modifications, have been associated with several neurological and developmental-delay disorders. Here, we report eight heterozygous de novo variants (one frameshift, two splice site, and five missense) in the gene encoding the BAF complex subunit double plant homeodomain finger 2 (DPF2). Affected individuals share common clinical features described in individuals with Coffin-Siris syndrome, including coarse facial features, global developmental delay, intellectual disability, speech impairment, and hypoplasia of fingernails and toenails. All variants occur within the highly conserved PHD1 and PHD2 motifs. Moreover, missense variants are situated close to zinc binding sites and are predicted to disrupt these sites. Pull-down assays of recombinant proteins and histone peptides revealed that a subset of the identified missense variants abolish or impaire DPF2 binding to unmodified and modified H3 histone tails. These results suggest an impairment of PHD finger structural integrity and cohesion and most likely an aberrant recognition of histone modifications. Furthermore, the overexpression of these variants in HEK293 and COS7 cell lines was associated with the formation of nuclear aggregates and the recruitment of both wild-type DPF2 and BRG1 to these aggregates. Expression analysis of truncating variants found in the affected individuals indicated that the aberrant transcripts escape nonsense-mediated decay. Altogether, we provide compelling evidence that de novo variants in DPF2 cause Coffin-Siris syndrome and propose a dominant-negative mechanism of pathogenicity.

EGFR controls IQGAP basolateral membrane localization and mitotic spindle orientation during epithelial morphogenesis.

Establishing the correct orientation of the mitotic spindle is an essential step in epithelial cell division in order to ensure that epithelial tubules form correctly during organ development and regeneration. While recent findings have identified some of the molecular mechanisms that underlie spindle orientation, many aspects of this process remain poorly understood. Here, we have used the 3D-MDCK model system to demonstrate a key role for a newly identified protein complex formed by IQGAP1 and the epithelial growth factor receptor (EGFR) in controlling the orientation of the mitotic spindle. IQGAP1 is a scaffolding protein that regulates many cellular pathways, from cell-cell adhesion to microtubule organization, and its localization in the basolateral membrane ensures correct spindle orientation. Through its IQ motifs, IQGAP1 binds to EGFR, which is responsible for maintaining IQGAP1 in the basolateral membrane domain. Silencing IQGAP1, or disrupting the basolateral localization of either IQGAP1 or EGFR, results in a non-polarized distribution of NuMA, mitotic spindle misorientation and defects in single lumen formation.

Novel Role of TRPML2 in the Regulation of the Innate Immune Response.

TRPMLs (or mucolipins) constitute a family of endosomal cation channels with homology to the transient receptor potential superfamily. In mammals, the TRPML family includes three members: TRPML1-3. Although TRPML1 and TRPML3 have been well characterized, the cellular function of TRPML2 has remained elusive. To address TRPML2 function in a physiologically relevant cell type, we first analyzed TRPML2 expression in different mouse tissues and organs and found that it was predominantly expressed in lymphoid organs and kidney. Quantitative RT-PCR revealed tight regulation of TRPML2 at the transcriptional level. Although TRPML2 expression was negligible in resting macrophages, TRPML2 mRNA and protein levels dramatically increased in response to TLR activation both in vitro and in vivo. Conversely, TRPML1 and TRPML3 levels did not change upon TLR activation. Immunofluorescence analysis demonstrated that endogenous TRPML2 primarily localized to recycling endosomes both in culture and primary cells, in contrast with TRPML1 and TRPML3, which distribute to the late and early endosomal pathway, respectively. To better understand the in vivo function of TRPML2, we generated a TRPML2-knockout mouse. We found that the production of several chemokines, in particular CCL2, was severely reduced in TRPML2-knockout mice. Furthermore, TRPML2-knockout mice displayed impaired recruitment of peripheral macrophages in response to i.p. injections of LPS or live bacteria, suggesting a potential defect in the immune response. Overall, our study reveals interesting differences in the regulation and distribution of the members of the TRPML family and identifies a novel role for TRPML2 in the innate immune response.

Lysine methyltransferase Smyd2 suppresses p53-dependent cardiomyocyte apoptosis.

Apoptosis, or programmed cell death, is an essential physiological process for proper embryogenesis as well as for homeostasis during aging. In addition, apoptosis is one of the major mechanisms causing cell loss in pathophysiological conditions such as heart failure. Thus, inhibition of apoptosis is an important approach for preventive and therapeutic strategies. Here we show that the histone 3 lysine 4- and lysine 36-specific methyltransferase Smyd2 acts as an endogenous antagonistic player of p53-dependent cardiomyocyte apoptosis. Smyd2 protein levels were significantly decreased in cardiomyocytes upon cobalt chloride-induced apoptosis or myocardial infarction, while p53 expression was enhanced. siRNA-mediated knockdown of Smyd2 in cultured cardiomyocytes further enhanced cobalt chloride-induced cardiomyocyte apoptosis. In contrast, Smyd2 overexpression resulted in marked methylation of p53 and prevented its accumulation as well as apoptotic cell death in an Hsp90-independent manner. Moreover, overexpression, of Smyd2, but not Smyd2Y240F lacking a methyl transferase activity, significantly rescued CoCl2-induced apoptosis in H9c2 cardioblasts. Finally, Smyd2 cardiomyocyte-specific deletion in vivo promoted apoptotic cell death upon myocardial infarction, which correlated with enhanced expression of p53 and pro-apoptotic Bax. Collectively, our data indicate Smyd2 as a cardioprotective protein by methylating p53.

C6orf89 encodes three distinct HDAC enhancers that function in the nucleolus, the golgi and the midbody.

We report here that C6orf89, which encodes a protein that interacts with bombesin receptor subtype-3 and accelerates cell cycle progression and wound repair in human bronchial epithelial cells (Liu et al., 2011, PLoS ONE 6: e23072), encodes one soluble and two type II membrane proteins that function as histone deacetylases (HDAC) enhancers. Soluble 34/64sp is selectively targeted to the nucleolus and is retained in nucleolar organiser regions (NORs) in mitotic cells. Nucleolar 34/64sp is integrated into the ribosomal gene transcription machinery, colocalises and coimmunoprecipitates with the Pol I transcription factor UBF, and undergoes a dramatic relocalisation to the nucleolus upon the arrest of rDNA transcription, protein synthesis and PI3K/mTORC2 signalling. Membrane 42/116mp localises to the Golgi and the midbody, and its controlled ectopic expression provokes the disruption of the Golgi cisternae and hinders the separation of daughter cells and the completion of mitosis. The latter effect is also produced by the microinjection of an affinity-purified amfion antibody. The identification of C60rf89 as a gene that encodes three distinct proteins with the capacity to enhance the activity of histone deacetylases (HDACs) in the nucleolus, the Golgi and the midbody provides new information regarding the components of the acetylome and their capacity to interact with different functional groups in the cell.

LAPTMs regulate lysosomal function and interact with mucolipin 1: new clues for understanding mucolipidosis type IV.

Loss-of-function mutations in mucolipin 1 (MCOLN1) result in mucolipidosis type IV (MLIV), a lysosomal storage disorder characterized by severe mental and psychomotor retardation. MCOLN1 is a lysosomal ion channel that belongs to the transient receptor potential (TRP) superfamily. To better understand the cellular function of MCOLN1, a split-ubiquitin yeast two-hybrid screen was performed with the purpose of revealing new MCOLN1 interaction partners. The screen identified two members of the lysosome-associated protein transmembrane (LAPTM) family as novel interaction partners of MCOLN1. The binding between MCOLN1 and LAPTM members (LAPTMs) was confirmed by co-immunoprecipitation and yeast two-hybrid assays. In addition, MCOLN1 and LAPTMs extensively colocalize at late endosomes and lysosomes. Overexpression of LAPTM4b caused enlargement of lysosomes and defective lysosomal degradation, indicating that LAPTMs are important for proper lysosomal function. Interestingly, lysosomal swelling induced by LAPTM4b was rescued by expression of MCOLN1, suggesting a functional connection between the two proteins. Finally, depletion of endogenous LAPTMs by siRNA induced accumulation of concentric multi-lamellar structures and electron-dense inclusions that closely resemble the structures found in MLIV cells. Overall, our data provide new insight into the molecular mechanisms of MCOLN1 function and suggest a potential role for LAPTMs in MLIV pathogenesis.

The focal adhesion protein ß-parvin controls cardiomyocyte shape and sarcomere assembly in response to mechanical load.

Physiological and pathological cardiac stress induced by exercise and hypertension, respectively, increase the hemodynamic load for the heart and trigger specific hypertrophic signals in cardiomyocytes leading to adaptive or maladaptive cardiac hypertrophy responses involving a mechanosensitive remodeling of the contractile cytoskeleton. Integrins sense load and have been implicated in cardiac hypertrophy, but how they discriminate between the two types of cardiac stress and translate mechanical loads into specific cytoskeletal signaling pathways is not clear. Here, we report that the focal adhesion protein ß-parvin is highly expressed in cardiomyocytes and facilitates the formation of cell protrusions, the serial assembly of newly synthesized sarcomeres, and the hypertrophic growth of neonatal rat ventricular cardiomyocytes (NRVCs) in vitro. In addition, physiological mechanical loading of NRVCs by either the application of cyclic, uni-axial stretch, or culture on physiologically stiff substrates promotes NRVC elongation in a ß-parvin-dependent manner, which is achieved by binding of ß-parvin to α/ß-PIX, which in turn activates Rac1. Importantly, loss-of-function studies in mice also revealed that ß-parvin is essential for the exercise-induced cardiac hypertrophy response in vivo. Our results identify ß-parvin as a novel mechano-responsive signaling hub in hypertrophic cardiomyocytes that drives cell elongation in response to physiological mechanical loads.

Alternative Splicing of Pericentrin Contributes to Cell Cycle Control in Cardiomyocytes.

Induction of cardiomyocyte proliferation is a promising option to regenerate the heart. Thus, it is important to elucidate mechanisms that contribute to the cell cycle arrest of mammalian cardiomyocytes. Here, we assessed the contribution of the pericentrin (Pcnt) S isoform to cell cycle arrest in postnatal cardiomyocytes. Immunofluorescence staining of Pcnt isoforms combined with SiRNA-mediated depletion indicates that Pcnt S preferentially localizes to the nuclear envelope, while the Pcnt B isoform is enriched at centrosomes. This is further supported by the localization of ectopically expressed FLAG-tagged Pcnt S and Pcnt B in postnatal cardiomyocytes. Analysis of centriole configuration upon Pcnt depletion revealed that Pcnt B but not Pcnt S is required for centriole cohesion. Importantly, ectopic expression of Pcnt S induced centriole splitting in a heterologous system, ARPE-19 cells, and was sufficient to impair DNA synthesis in C2C12 myoblasts. Moreover, Pcnt S depletion enhanced serum-induced cell cycle re-entry in postnatal cardiomyocytes. Analysis of mitosis, binucleation rate, and cell number suggests that Pcnt S depletion enhances serum-induced progression of postnatal cardiomyocytes through the cell cycle resulting in cell division. Collectively, our data indicate that alternative splicing of Pcnt contributes to the establishment of cardiomyocyte cell cycle arrest shortly after birth.

Myogenin controls via AKAP6 non-centrosomal microtubule-organizing center formation at the nuclear envelope.

Non-centrosomal microtubule-organizing centers (MTOCs) are pivotal for the function of multiple cell types, but the processes initiating their formation are unknown. Here, we find that the transcription factor myogenin is required in murine myoblasts for the localization of MTOC proteins to the nuclear envelope. Moreover, myogenin is sufficient in fibroblasts for nuclear envelope MTOC (NE-MTOC) formation and centrosome attenuation. Bioinformatics combined with loss- and gain-of-function experiments identified induction of AKAP6 expression as one central mechanism for myogenin-mediated NE-MTOC formation. Promoter studies indicate that myogenin preferentially induces the transcription of muscle- and NE-MTOC-specific isoforms of Akap6 and Syne1, which encodes nesprin-1α, the NE-MTOC anchor protein in muscle cells. Overexpression of AKAP6ß and nesprin-1α was sufficient to recruit endogenous MTOC proteins to the nuclear envelope of myoblasts in the absence of myogenin. Taken together, our results illuminate how mammals transcriptionally control the switch from a centrosomal MTOC to an NE-MTOC and identify AKAP6 as a novel NE-MTOC component in muscle cells.

IQGAP3, a YAP Target, Is Required for Proper Cell-Cycle Progression and Genome Stability.

Controlling cell proliferation is critical for organism development, tissue homeostasis, disease, and regeneration. IQGAP3 has been shown to be required for proper cell proliferation and migration, and is associated to a number of cancers. Moreover, its expression is inversely correlated with the overall survival rate in the majority of cancers. Here, we show that IQGAP3 expression is elevated in cervical cancer and that in these cancers IQGAP3 high expression is correlated with an increased lethality. Furthermore, we demonstrate that IQGAP3 is a target of YAP, a regulator of cell cycle gene expression. IQGAP3 knockdown resulted in an increased percentage of HeLa cells in S phase, delayed progression through mitosis, and caused multipolar spindle formation and consequentially aneuploidy. Protein-protein interaction studies revealed that IQGAP3 interacts with MMS19, which is known in Drosophila to permit, by competitive binding to Xpd, Cdk7 to be fully active as a Cdk-activating kinase (CAK). Notably, IQGAP3 knockdown caused decreased MMS19 protein levels and XPD knockdown partially rescued the reduced proliferation rate upon IQGAP3 knockdown. This suggests that IQGAP3 modulates the cell cycle via the MMS19/XPD/CAK axis. Thus, in addition to governing proliferation and migration, IQGAP3 is a critical regulator of mitotic progression and genome stability. IMPLICATIONS: Our data indicate that, while IQGAP3 inhibition might be initially effective in decreasing cancer cell proliferation, this approach harbors the risk to promote aneuploidy and, therefore, the formation of more aggressive cancers.

Identification of the penta-EF-hand protein ALG-2 as a Ca2+-dependent interactor of mucolipin-1.

Loss of function mutations in mucolipin-1 (MCOLN1) have been linked to mucolipidosis type IV (MLIV), a recessive lysosomal storage disease characterized by severe neurological and ophthalmological abnormalities. MCOLN1 is an ion channel that regulates membrane transport along the endolysosomal pathway. It has been suggested that MCOLN1 participates in several Ca(2+)-dependent processes, including fusion of lysosomes with the plasma membrane, fusion of late endosomes and autophagosomes with lysosomes, and lysosomal biogenesis. Here, we searched for proteins that interact with MCOLN1 in a Ca(2+)-dependent manner. We found that the penta-EF-hand protein ALG-2 binds to the NH-terminal cytosolic tail of MCOLN1. The interaction is direct, strictly dependent on Ca(2+), and mediated by a patch of charged and hydrophobic residues located between MCOLN1 residues 37 and 49. We further show that MCOLN1 and ALG-2 co-localize to enlarged endosomes induced by overexpression of an ATPase-defective dominant-negative form of Vps4B (Vps4B(E235Q)). In agreement with the proposed role of MCOLN1 in the regulation of fusion/fission events, we found that overexpression of MCOLN1 caused accumulation of enlarged, aberrant endosomes that contain both early and late endosome markers. Interestingly, aggregation of abnormal endosomes was greatly reduced when the ALG-2-binding domain in MCOLN1 was mutated, suggesting that ALG-2 regulates MCOLN1 function. Overall, our data provide new insight into the molecular mechanisms that regulate MCOLN1 activity. We propose that ALG-2 acts as a Ca(2+) sensor that modulates the function of MCOLN1 along the late endosomal-lysosomal pathway.

Autophagic dysfunction in mucolipidosis type IV patients.

Mutations in Mucolipin 1 (MCOLN1) have been linked to mucolipidosis type IV (MLIV), a lysosomal storage disease characterized by several neurological and ophthalmological abnormalities. It has been proposed that MCOLN1 might regulate transport of membrane components in the late endosomal-lysosomal pathway; however, the mechanisms by which defects of MCOLN1 function result in mental and psychomotor retardation remain largely unknown. In this study, we show constitutive activation of autophagy in fibroblasts obtained from MLIV patients. Accumulation of autophagosomes in MLIV cells was due to the increased de novo autophagosome formation and to delayed fusion of autophagosomes with late endosomes/lysosomes. Impairment of the autophagic pathway led to increased levels and aggregation of p62, suggesting that abnormal accumulation of ubiquitin proteins may contribute to the neurodegeneration observed in MLIV patients. In addition, we found that delivery of platelet-derived growth factor receptor to lysosomes is delayed in MCOLN1-deficient cells, suggesting that MCOLN1 is necessary for efficient fusion of both autophagosomes and late endosomes with lysosomes. Our data are in agreement with recent evidence showing that autophagic defects may be a common characteristic of many neurodegenerative disorders.

AKAP6 orchestrates the nuclear envelope microtubule-organizing center by linking golgi and nucleus via AKAP9.

The switch from centrosomal microtubule-organizing centers (MTOCs) to non-centrosomal MTOCs during differentiation is poorly understood. Here, we identify AKAP6 as key component of the nuclear envelope MTOC. In rat cardiomyocytes, AKAP6 anchors centrosomal proteins to the nuclear envelope through its spectrin repeats, acting as an adaptor between nesprin-1α and Pcnt or AKAP9. In addition, AKAP6 and AKAP9 form a protein platform tethering the Golgi to the nucleus. Both Golgi and nuclear envelope exhibit MTOC activity utilizing either AKAP9, or Pcnt-AKAP9, respectively. AKAP6 is also required for formation and activity of the nuclear envelope MTOC in human osteoclasts. Moreover, ectopic expression of AKAP6 in epithelial cells is sufficient to recruit endogenous centrosomal proteins. Finally, AKAP6 is required for cardiomyocyte hypertrophy and osteoclast bone resorption activity. Collectively, we decipher the MTOC at the nuclear envelope as a bi-layered structure generating two pools of microtubules with AKAP6 as a key organizer.

Daxx functions as a scaffold of a protein assembly constituted by GLUT4, JNK1 and KIF5B.

We have previously reported the physical interaction between Daxx, the adaptor protein that mediates activation of the Jun amino-terminal kinase (JNK), and GLUT4, the insulin-dependent glucose transporter, interaction that involves their C-domains. Co-immunoprecipitation and two-hybrid-based protein-protein interaction studies show now that Daxx and GLUT4 interact with JNK1 through D-sites in their NH(2)-(aa 1-501) and large endofacial loop, respectively. Serum deprivation strongly enhances the association of JNK1 with Daxx and dissociates the kinase from GLUT4. SP600125, a potent JNK1 inhibitor, reduces the JNK1 activity associated with GLUT4 and the phosphorylation of two minor GLUT4 species in serum-starved 3T3-L1 adipocytes. In addition, Daxx interacts with kinesin KIF5B through the 6xTPR domain of the kinesin light chain, a domain engaged in the grab hold of protein cargo by kinesin motors that codistribute with JNK. Depletion of Daxx in 3T3-L1 adipocytes provokes the partial translocation of the GLUT4 retained in the GLUT4 storage compartment to endosomes.

Mucolipin 1 channel activity is regulated by protein kinase A-mediated phosphorylation.

Mucolipins constitute a family of cation channels with homology with the transient receptor potential family. Mutations in MCOLN1 (mucolipin 1) have been linked to mucolipidosis type IV, a recessive lysosomal storage disease characterized by severe neurological and ophthalmologic abnormalities. At present, little is known about the mechanisms that regulate MCOLN1 activity. In the present paper, we addressed whether MCOLN1 activity is regulated by phosphorylation. We identified two PKA (protein kinase A) consensus motifs in the C-terminal tail of MCOLN1, containing Ser(557) and Ser(559). Ser(557) was the principal phosphorylation site, as mutation of this residue to alanine caused a greater than 75% reduction in the total levels of phosphorylated MCOLN1 C-terminal tail. Activation of PKA with forskolin promoted MCOLN1 phosphorylation, both in vitro and in vivo. In contrast, addition of the PKA inhibitor H89 abolished MCOLN1 phosphorylation. We also found that PKA-mediated phosphorylation regulates MCOLN1 channel activity. Forskolin treatment decreased MCOLN1 channel activity, whereas treatment with H89 increased MCOLN1 channel activity. The stimulatory effect of H89 on MCOLN1 function was not observed when Ser(557) and Ser(559) were mutated to alanine residues, indicating that these two residues are essential for PKA-mediated negative regulation of MCOLN1. This paper presents the first example of regulation of a member of the mucolipin family by phosphorylation.

Developmental alterations in centrosome integrity contribute to the post-mitotic state of mammalian cardiomyocytes.

Mammalian cardiomyocytes become post-mitotic shortly after birth. Understanding how this occurs is highly relevant to cardiac regenerative therapy. Yet, how cardiomyocytes achieve and maintain a post-mitotic state is unknown. Here, we show that cardiomyocyte centrosome integrity is lost shortly after birth. This is coupled with relocalization of various centrosome proteins to the nuclear envelope. Consequently, postnatal cardiomyocytes are unable to undergo ciliogenesis and the nuclear envelope adopts the function as cellular microtubule organizing center. Loss of centrosome integrity is associated with, and can promote, cardiomyocyte G0/G1 cell cycle arrest suggesting that centrosome disassembly is developmentally utilized to achieve the post-mitotic state in mammalian cardiomyocytes. Adult cardiomyocytes of zebrafish and newt, which are able to proliferate, maintain centrosome integrity. Collectively, our data provide a novel mechanism underlying the post-mitotic state of mammalian cardiomyocytes as well as a potential explanation for why zebrafish and newts, but not mammals, can regenerate their heart.

Synaptotagmin-like proteins control the formation of a single apical membrane domain in epithelial cells.

The formation of epithelial tissues requires both the generation of apical-basal polarity and the coordination of this polarity between neighbouring cells to form a central lumen. During de novo lumen formation, vectorial membrane transport contributes to the formation of a singular apical membrane, resulting in the contribution of each cell to only a single lumen. Here, from a functional screen for genes required for three-dimensional epithelial architecture, we identify key roles for synaptotagmin-like proteins 2-a and 4-a (Slp2-a/4-a) in the generation of a single apical surface per cell. Slp2-a localizes to the luminal membrane in a PtdIns(4,5)P(2)-dependent manner, where it targets Rab27-loaded vesicles to initiate a single lumen. Vesicle tethering and fusion is controlled by Slp4-a, in conjunction with Rab27/Rab3/Rab8 and the SNARE syntaxin-3. Together, Slp2-a/4-a coordinate the spatiotemporal organization of vectorial apical transport to ensure that only a single apical surface, and thus the formation of a single lumen, occurs per cell.

The Cdc42 GEF Intersectin 2 controls mitotic spindle orientation to form the lumen during epithelial morphogenesis.

Epithelial organs are made of tubes and cavities lined by a monolayer of polarized cells that enclose the central lumen. Lumen formation is a crucial step in the formation of epithelial organs. The Rho guanosine triphosphatase (GTPase) Cdc42, which is a master regulator of cell polarity, regulates the formation of the central lumen in epithelial morphogenesis. However, how Cdc42 is regulated during this process is still poorly understood. Guanine nucleotide exchange factors (GEFs) control the activation of small GTPases. Using the three-dimensional Madin-Darby canine kidney model, we have identified a Cdc42-specific GEF, Intersectin 2 (ITSN2), which localizes to the centrosomes and regulates Cdc42 activation during epithelial morphogenesis. Silencing of either Cdc42 or ITSN2 disrupts the correct orientation of the mitotic spindle and normal lumen formation, suggesting a direct relationship between these processes. Furthermore, we demonstrated this direct relationship using LGN, a component of the machinery for mitotic spindle positioning, whose disruption also results in lumen formation defects.

Mucolipidosis type IV: the importance of functional lysosomes for efficient autophagy.

Mucolipidosis IV (MLIV) is a lysosomal storage disorder characterized by severe neurological and ophthalmologic abnormalities. In contrast with most lysosomal storage disorders, which are attributed to the absence of specific lysosomal hydrolases, accumulation of material in MLIV results from defects in membrane transport along the late endocytic pathway. Mutations in MCOLN1 are the cause of MLIV; however, how the lack of MCOLN1 function ultimately leads to neurodegeneration remains largely unknown. We found that MCOLN1 is required for efficient fusion of both late endosomes and autophagosomes with lysosomes. Impaired autophagosome degradation results in accumulation of autophagosomes in MLIV fibroblasts. In addition, we found increased levels and aggregation of p62, suggesting that abnormal accumulation of ubiquitinated protein inclusions may contribute to the neurodegenerative phenotype observed in MLIV patients. These findings corroborate recent evidence indicating that defects in autophagy may be a common feature of many neurodegenerative disorders.

Two di-leucine motifs regulate trafficking of mucolipin-1 to lysosomes.

Mutations in the mucolipin-1 gene have been linked to mucolipidosis type IV, a lysosomal storage disorder characterized by severe neurological and ophthalmologic abnormalities. Mucolipin-1 is a membrane protein containing six putative transmembrane domains with both its N- and C-termini localized facing the cytosol. To gain information on the sorting motifs that mediate the trafficking of this protein to lysosomes, we have generated chimeras in which the N- and C- terminal tail portions of mucolipin-1 were fused to a reporter gene. In this article, we report the identification of two separate di-leucine-type motifs that co-operate to regulate the transport of mucolipin-1 to lysosomes. One di-leucine motif is positioned at the N-terminal cytosolic tail and mediates direct transport to lysosomes, whereas the other di-leucine motif is found at the C-terminal tail and functions as an adaptor protein 2-dependent internalization motif. We have also found that the C-terminal tail of mucolipin-1 is palmitoylated and that this modification might regulate the efficiency of endocytosis. Finally, the mutagenesis of both di-leucine motifs abrogated lysosomal accumulation and resulted in cell-surface redistribution of mucolipin-1. Taken together, these results reveal novel information regarding the motifs that regulate mucolipin-1 trafficking and suggest a role for palmitoylation in protein sorting.