Potential tumor suppressor. Seems to be involved in lymphocyte adhesion by linking RAP1A activation upon T-cell receptor or chemokine stimulation to integrin activation. Isoform 2 stimulates lymphocyte polarization and the patch-like distribution of ITGAL/LFA-1, resulting in an enhanced adhesion to ICAM1. Together with RAP1A may participate in regulation of microtubule growth. The association of isoform 2 with activated RAP1A is required for directional movement of endothelial cells during wound healing (By similarity). May be involved in regulation of Ras apoptotic function. The RASSF5-STK4/MST1 complex may mediate HRAS and KRAS induced apoptosis. Interacts directly with activated HRAS; a RASSF5-STK4/MST1 complex probably associates with activated HRAS (PubMed:9488663, PubMed:11857081, PubMed:11864565, PubMed:18596699). Interacts with KRAS (PubMed:11857081, PubMed:11864565). Probably interacts with Ras-like GTPases RRAS, MRAS, RAP1B, RAP2A and RALA (PubMed:11857081). Interacts with RRAS2 (By similarity). Can self-associate (PubMed:11857081). Interacts with RSSF1 isoform A (PubMed:11857081). The RSSF1 isoform A-RSSF5 heterodimer probably mediates the association of RSSF1 with HRAS (PubMed:11864565, PubMed:11857081). Isoform 2 interacts with activated RAP1A and ITGAL/LFA-1 (By similarity). Binds STK4/MST1, inhibiting STK4/MST1 autoactivation (PubMed:11864565). Isoform 2 is mainly located in the perinuclear region of unstimulated primary T-cells. Upon stimulation translocates to the leading edge and colocalizes with ITGAL/LFA-1 in the peripheral zone of the immunological synapse. Isoform 2 is localized to growing microtubules in vascular endothelial cells and is dissociated from microtubules by activated RAP1A (By similarity). Extended N-terminus.