Type I phosphatidylinositol 4-phosphate 5-kinase homo- and heterodimerization determines its membrane localization and activity.

Type I phosphatidylinositol 4-phosphate 5-kinases (PIP5KIs; α, ß, and γ) are a family of isoenzymes that produce phosphatidylinositol 4,5-bisphosphate [PI(4,5)P2] using phosphatidylinositol 4-phosphate as substrate. Their structural homology with the class II lipid kinases [type II phosphatidylinositol 5-phosphate 4-kinase (PIP4KII)] suggests that PIP5KI dimerizes, although this has not been formally demonstrated. Neither the hypothetical structural dimerization determinants nor the functional consequences of dimerization have been studied. Here, we used Förster resonance energy transfer, coprecipitation, and ELISA to show that PIP5KIß forms homo- and heterodimers with PIP5KIγ_i2 in vitro and in live human cells. Dimerization appears to be a general phenomenon for PIP5KI isoenzymes because PIP5KIß/PIP5KIα heterodimers were also detected by mass spectrometry. Dimerization was independent of actin cytoskeleton remodeling and was also observed using purified proteins. Mutagenesis studies of PIP5KIß located the dimerization motif at the N terminus, in a region homologous to that implicated in PIP4KII dimerization. PIP5KIß mutants whose dimerization was impaired showed a severe decrease in PI(4,5)P2 production and plasma membrane delocalization, although their association to lipid monolayers was unaltered. Our results identify dimerization as an integral feature of PIP5K proteins and a central determinant of their enzyme activity.

Functional characterization of NIPBL physiological splice variants and eight splicing mutations in patients with Cornelia de Lange syndrome.

Cornelia de Lange syndrome (CdLS) is a congenital developmental disorder characterized by distinctive craniofacial features, growth retardation, cognitive impairment, limb defects, hirsutism, and multisystem involvement. Mutations in five genes encoding structural components (SMC1A, SMC3, RAD21) or functionally associated factors (NIPBL, HDAC8) of the cohesin complex have been found in patients with CdLS. In about 60% of the patients, mutations in NIPBL could be identified. Interestingly, 17% of them are predicted to change normal splicing, however, detailed molecular investigations are often missing. Here, we report the first systematic study of the physiological splicing of the NIPBL gene, that would reveal the identification of four new splicing isoforms ΔE10, ΔE12, ΔE33,34, and B'. Furthermore, we have investigated nine mutations affecting splice-sites in the NIPBL gene identified in twelve CdLS patients. All mutations have been examined on the DNA and RNA level, as well as by in silico analyses. Although patients with mutations affecting NIPBL splicing show a broad clinical variability, the more severe phenotypes seem to be associated with aberrant transcripts resulting in a shift of the reading frame.