The tumor suppressor gene DLC1 encodes a multi-domain protein, which includes a Rho-GAP domain (Rho GTPase activating protein domain) that negatively regulates the activity of RhoA, B, and C, and has been hypothesized to be the basis of its tumor suppressor functions. DLC1 also contains a less well characterized START (StAR-related lipid transfer) domain in its C-terminus, whose overall contribution to DLC1 function remains incompletely understood. START domains in other proteins have been shown to bind lipids, but no lipid has yet been identified that binds the DLC1 START domain. In the present study, we attempted to gain further insight into the molecular function and lipid binding properties of the DLC1 START domain.

We previously determined that Caveolin-1, the main structural component and marker of caveolae, interacts with the DLC1 START domain, and this interaction contributes to the full tumor suppressor activity of DLC1. In unpublished studies, we additionally identified Phospholipase C delta 1 (PLCD1), which participates in intracellular Ca2+ mobilization, forming a complex with the DLC1 START domain and with Caveolin-1.

We have now developed evidence that, in addition to binding Caveolin-1 and PLCD1, the DLC1 START domain binds a lipid, phosphatidylserine (PS), and have identified a colon cancer-associated DLC1-START domain mutant (R947C) that results in a protein deficient in forming a complex with all three binding partners and displays reduced tumor suppressor activity but intact RhoGAP activity. The interaction between the DLC1-START domain and PS can occur independently of PLCD1 and/or Caveolin-1, but PS promotes complex formation between the DLC1 START domain and PLCD1 or Caveolin-1, without altering PLCD1 to Caveolin-1 binding. Biological assays with wild type DLC1 show cooperation between DLC1 and PLCD1 or Caveolin-1 to inhibit cell migration, but no cooperation is observed when all three genes are overexpressed, suggesting that Caveolin-1 and PLCD1 occupy the same migration signaling pathway. Thus, we have identified a biologically relevant complex between DLC1 and three macromolecules (PLCD1, PS and Caveolin-1) that contributes to the full tumor suppressor function of DLC1 independently of its RhoGAP activity, and are testing the hypothesis that PS serves as a linker between the DLC1 START domain and PLCD1 or Caveolin-1.

Citation Format: Beatriz Sanchez-Solana, Dunrui Wang, Xiaolan Qian, Alex Papageorge, Parthibane Velayoudame, Jairaj Acharya, Douglas R. Lowy. The START domain of the DLC1 tumor suppressor binds phosphatidylserine and two proteins, PLCD1 and Caveolin-1 [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2019; 2019 Mar 29-Apr 3; Atlanta, GA. Philadelphia (PA): AACR; Cancer Res 2019;79(13 Suppl):Abstract nr 1762.