Structural basis for the distinct roles of non-conserved Pro116 and conserved Tyr124 of BCH domain of yeast p50RhoGAP.

p50RhoGAP is a key protein that interacts with and downregulates the small GTPase RhoA. p50RhoGAP is a multifunctional protein containing the BNIP-2 and Cdc42GAP Homology (BCH) domain that facilitates protein-protein interactions and lipid binding and the GAP domain that regulates active RhoA population. We recently solved the structure of the BCH domain from yeast p50RhoGAP (YBCH) and showed that it maintains the adjacent GAP domain in an auto-inhibited state through the ß5 strand. Our previous WT YBCH structure shows that a unique kink at position 116 thought to be made by a proline residue between alpha helices α6 and α7 is essential for the formation of intertwined dimer from asymmetric monomers. Here we sought to establish the role and impact of this Pro116. However, the kink persists in the structure of P116A mutant YBCH domain, suggesting that the scaffold is not dictated by the proline residue at this position. We further identified Tyr124 (or Tyr188 in HBCH) as a conserved residue in the crucial ß5 strand. Extending to the human ortholog, when substituted to acidic residues, Tyr188D or Tyr188E, we observed an increase in RhoA binding and self-dimerization, indicative of a loss of inhibition of the GAP domain by the BCH domain. These results point to distinct roles and impact of the non-conserved and conserved amino acid positions in regulating the structural and functional complexity of the BCH domain.

PRUNE2 inhibits progression of colorectal cancer in vitro and in vivo.

Prune homolog 2 with BCH domain (PRUNE2) is associated with prostate cancer, neuroblastoma, glioblastoma and melanoma; however, the function of PRUNE2 in colorectal cancer (CRC) remains unknown. The present study aimed to evaluate the effects of PRUNE2 on the development of CRC. The biological function of PRUNE2 in CRC cell lines was investigated by using Cell Counting Kit-8, colony formation, flow cytometry and Transwell assay. Additionally, a mouse model was established to investigate the effect of PRUNE2 on metastasis of CRC cells. The expression levels of PRUNE2 were lower in CRC compared with adjacent normal tissue and this expression pattern was associated with poor relapse-free survival probability. PRUNE2 overexpression significantly decreased cell proliferation and invasion, increased cell apoptosis and arrested the cell cycle. Consistently, it increased the protein expression levels of pro-apoptosis genes and decreased the expression of antiapoptotic proteins. PRUNE2 knockdown had the opposite effects. Furthermore, PRUNE2 overexpression decreased the tumorigenicity of CRC cells. In conclusion, PRUNE2 decreased cell survival, proliferation, invasion and tumorigenicity and promoted apoptosis, suggesting that PRUNE2 may function as a tumor-suppressive gene in CRC.

Structural basis for p50RhoGAP BCH domain-mediated regulation of Rho inactivation.

Spatiotemporal regulation of signaling cascades is crucial for various biological pathways, under the control of a range of scaffolding proteins. The BNIP-2 and Cdc42GAP Homology (BCH) domain is a highly conserved module that targets small GTPases and their regulators. Proteins bearing BCH domains are key for driving cell elongation, retraction, membrane protrusion, and other aspects of active morphogenesis during cell migration, myoblast differentiation, and neuritogenesis. We previously showed that the BCH domain of p50RhoGAP (ARHGAP1) sequesters RhoA from inactivation by its adjacent GAP domain; however, the underlying molecular mechanism for RhoA inactivation by p50RhoGAP remains unknown. Here, we report the crystal structure of the BCH domain of p50RhoGAP Schizosaccharomyces pombe and model the human p50RhoGAP BCH domain to understand its regulatory function using in vitro and cell line studies. We show that the BCH domain adopts an intertwined dimeric structure with asymmetric monomers and harbors a unique RhoA-binding loop and a lipid-binding pocket that anchors prenylated RhoA. Interestingly, the ß5-strand of the BCH domain is involved in an intermolecular ß-sheet, which is crucial for inhibition of the adjacent GAP domain. A destabilizing mutation in the ß5-strand triggers the release of the GAP domain from autoinhibition. This renders p50RhoGAP active, thereby leading to RhoA inactivation and increased self-association of p50RhoGAP molecules via their BCH domains. Our results offer key insight into the concerted spatiotemporal regulation of Rho activity by BCH domain-containing proteins.