Two novel lipases purified from rice bran displaying lipolytic and esterification activities.

In this study, two novel lipases, rice bran lipase 1 (RBL1) and rice bran lipase 2 (RBL2), were first identified in rice (Oryza sativa) bran. Through the purification by ammonium sulfate precipitation, ion-exchange chromatography and size-exclusion chromatography, RBL1 and RBL2 were purified to 36- and 339-fold with the final specific activity of 4.73 and 44.06 U/mg, respectively. The purified RBL1 and RBL2 had the molecular weight of 18.8 and 35.5 kDa, respectively. The Km values of RBL1 and RBL2 were 0.766 and 0.432 mM while catalytic efficiency (kcat/Km) values of RBL1 and RBL2 were 102.4 and 2559.3 s-1/mM, respectively. RBL1 and RBL2 both showed esterification activity, but had no transesterification activity. In a solvent-free system, RBL1 and RBL2 exhibited degree of esterification (ED) of 4.4% and 6.5%, respectively. These two novel lipases exerted great properties for their potentials in industrial applications. First, RBL1 and RBL2 showed both mild reaction pH of 7.0 and temperature of 35 °C and 50 °C, respectively. Secondly, they showed great tolerance to several organic solvents and detergents while RBL1 exhibited great pH stability across a very broad range of pH (pH 3-12). Lastly and most importantly, RBL1 and RBL2 both exhibited esterification activity better than a commercial lipase, Candida rugosa lipase (CRL), in a solvent-free system. In conclusion, two novel lipases, RBL1 and RBL2, are different from published native lipases in rice bran and may be alternative potential candidates of biocatalysts contributing to the development of diverse industrial application fields.

Conservation and divergence of C-terminal domain structure in the retinoblastoma protein family.

The retinoblastoma protein (Rb) and the homologous pocket proteins p107 and p130 negatively regulate cell proliferation by binding and inhibiting members of the E2F transcription factor family. The structural features that distinguish Rb from other pocket proteins have been unclear but are critical for understanding their functional diversity and determining why Rb has unique tumor suppressor activities. We describe here important differences in how the Rb and p107 C-terminal domains (CTDs) associate with the coiled-coil and marked-box domains (CMs) of E2Fs. We find that although CTD-CM binding is conserved across protein families, Rb and p107 CTDs show clear preferences for different E2Fs. A crystal structure of the p107 CTD bound to E2F5 and its dimer partner DP1 reveals the molecular basis for pocket protein-E2F binding specificity and how cyclin-dependent kinases differentially regulate pocket proteins through CTD phosphorylation. Our structural and biochemical data together with phylogenetic analyses of Rb and E2F proteins support the conclusion that Rb evolved specific structural motifs that confer its unique capacity to bind with high affinity those E2Fs that are the most potent activators of the cell cycle.

Structural Conservation and E2F Binding Specificity within the Retinoblastoma Pocket Protein Family.

The human pocket proteins retinoblastoma (Rb), p107, and p130 are critical negative regulators of the cell cycle and contribute to tumor suppression. While strong structural conservation within the pocket protein family provides for some functional redundancy, important differences have been observed and may underlie the reason that Rb is a uniquely potent tumor suppressor. It has been proposed that distinct pocket protein activities are mediated by their different E2F transcription factor binding partners. In humans, Rb binds E2F1-E2F5, whereas p107 and p130 almost exclusively associate with E2F4 and E2F5. To identify the molecular determinants of this specificity, we compared the crystal structures of Rb and p107 pocket domains and identified several key residues that contribute to E2F selectivity in the pocket family. Mutation of these residues in p107 to match the analogous residue in Rb results in an increase in affinity for E2F1 and E2F2 and an increase in the ability of p107 to inhibit E2F2 transactivation. Additionally, we investigated how phosphorylation by Cyclin-dependent kinase on distinct residues regulates p107 affinity for the E2F4 transactivation domain. We found that phosphorylation of residues S650 and S975 weakens the E2F4 transactivation domain binding. Our data reveal molecular features of pocket proteins that are responsible for their similarities and differences in function and regulation.

Structural mechanisms of DREAM complex assembly and regulation.

The DREAM complex represses cell cycle genes during quiescence through scaffolding MuvB proteins with E2F4/5 and the Rb tumor suppressor paralog p107 or p130. Upon cell cycle entry, MuvB dissociates from p107/p130 and recruits B-Myb and FoxM1 for up-regulating mitotic gene expression. To understand the biochemical mechanisms underpinning DREAM function and regulation, we investigated the structural basis for DREAM assembly. We identified a sequence in the MuvB component LIN52 that binds directly to the pocket domains of p107 and p130 when phosphorylated on the DYRK1A kinase site S28. A crystal structure of the LIN52-p107 complex reveals that LIN52 uses a suboptimal LxSxExL sequence together with the phosphate at nearby S28 to bind the LxCxE cleft of the pocket domain with high affinity. The structure explains the specificity for p107/p130 over Rb in the DREAM complex and how the complex is disrupted by viral oncoproteins. Based on insights from the structure, we addressed how DREAM is disassembled upon cell cycle entry. We found that p130 and B-Myb can both bind the core MuvB complex simultaneously but that cyclin-dependent kinase phosphorylation of p130 weakens its association. Together, our data inform a novel target interface for studying MuvB and p130 function and the design of inhibitors that prevent tumor escape in quiescence.

Tandem E2F binding sites in the promoter of the p107 cell cycle regulator control p107 expression and its cellular functions.

The retinoblastoma tumor suppressor (Rb) is a potent and ubiquitously expressed cell cycle regulator, but patients with a germline Rb mutation develop a very specific tumor spectrum. This surprising observation raises the possibility that mechanisms that compensate for loss of Rb function are present or activated in many cell types. In particular, p107, a protein related to Rb, has been shown to functionally overlap for loss of Rb in several cellular contexts. To investigate the mechanisms underlying this functional redundancy between Rb and p107 in vivo, we used gene targeting in embryonic stem cells to engineer point mutations in two consensus E2F binding sites in the endogenous p107 promoter. Analysis of normal and mutant cells by gene expression and chromatin immunoprecipitation assays showed that members of the Rb and E2F families directly bound these two sites. Furthermore, we found that these two E2F sites controlled both the repression of p107 in quiescent cells and also its activation in cycling cells, as well as in Rb mutant cells. Cell cycle assays further indicated that activation of p107 transcription during S phase through the two E2F binding sites was critical for controlled cell cycle progression, uncovering a specific role for p107 to slow proliferation in mammalian cells. Direct transcriptional repression of p107 by Rb and E2F family members provides a molecular mechanism for a critical negative feedback loop during cell cycle progression and tumorigenesis. These experiments also suggest novel therapeutic strategies to increase the p107 levels in tumor cells.

Cyclin B and cyclin A confer different substrate recognition properties on CDK2.

The transitions of the cell cycle are regulated by the cyclin dependent protein kinases (CDKs). The cyclins activate their respective CDKs and confer substrate recognition properties. We report the structure of phospho-CDK2/cyclin B and show that cyclin B confers M phase-like properties on CDK2, the kinase that is usually associated with S phase. Cyclin B produces an almost identical activated conformation of CDK2 as that produced by cyclin A. There are differences between cyclin A and cyclin B at the recruitment site, which in cyclin A is used to recruit substrates containing an RXL motif. Because of sequence differences this site in cyclin B binds RXL motifs more weakly than in cyclin A. Despite similarity in kinase structures, phospho-CDK2/cyclin B phosphorylates substrates, such as nuclear lamin and a model peptide derived from p107, at sequences SPXX that differ from the canonical CDK2/cyclin A substrate recognition motif, SPXK. CDK2/cyclin B phosphorylation at these non-canonical sites is not dependent on the presence of a RXL recruitment motif. The p107 peptide contains two SP motifs each followed by a non-canonical sequence of which only one site (Ser640) is phosphorylated by pCDK2/cyclin A while two sites are phosphorylated by pCDK2/cyclin B. The second site is too close to the RXL motif to allow the cyclin A recruitment site to be effective, as previous work has shown that there must be at least 16 residues between the catalytic site serine and the RXL motif. Thus the cyclins A and B in addition to their role in promoting the activatory conformational switch in CDK2, also provide differential substrate specificity.