These interactions may also regulate ESCRT-III stability and/or disassembly CT99021 clinical trial because interactions between Bro1p and Snf7p can enhance the stability of ESCRT-III assemblies and inhibit their disassembly by Vps4p. In addition to ALIX, the human proteome contains four other known Bro1 domain-containing proteins: HD-PTP, BROX, RHPN1 and RHPN2. HD-PTP and ALIX share a similar domain organization, except that HD-PTP also contains an additional C-terminal protein tyrosine phosphatase-like domain. HD-PTP appears to function as the Bro1p homolog for ESCRT-dependent protein sorting in mammalian MVB pathways. The function of BROX is less clear, but this protein also likely functions in ESCRT-dependent membrane remodeling processes because BROX also binds CHMP4 and, like other ESCRT proteins, becomes partially trapped on endosomal membranes upon overexpression of a dominant negative mutant of VPS4B. BROX comprises a single Bro1 domain and has a C-terminal “CAAX” farnesylation site. The two human rhophilin proteins, RHPN1 and RHPN2, are Rho-GTP binding proteins involved in cytoskeletal dynamics. The full length RHPN2 protein reportedly does not bind CHMP4A, and neither protein has been clearly linked to the ESCRT pathway, although RHPN2 localizes to late endosomes and this localization is mediated by the protein’s Bro1 domain. In addition to connecting membrane-specific adaptors to the downstream ESCRT-III fission machinery, there are indications that Bro1 domains may make additional interactions that promote cargo sorting and membrane remodeling. For example, the Bro1 domains of ALIX, HD-PTP, BROX, and RHPN1 can bind the NC domains of HIV-1 Gag proteins and stimulate the release of virus-like particles. Moreover, even BROX mutants that cannot bind CHMP4 retain some ability to stimulate virus budding, indicating that Bro1 domains can also act in other ways. One possibility is that Bro1 domains may bind membranes and induce negative curvature. This model was first proposed because the crystal structure of the Bro1 domain of yeast Bro1p revealed a basic convex surface on the elongated, banana-shaped domain that could mediate membrane binding and thereby induce membrane curvature. Although direct experimental evidence is lacking, analogous membrane bending activities have been well documented for BAR domains, which bind membranes using curved basic surfaces. Furthermore, the ability to promote negative membrane curvature could explain how highly divergent Bro1 domains can all promote the budding of minimal HIV-1 Gag constructs. To date, high-resolution structural information has only been available for the Bro1 domains from yeast Bro1p and the human protein ALIX. We reasoned that comparing additional Bro1 domain structures might help to identify key architectural features and we have therefore determined the crystal structure of human BROX. This new structure allowed us to identify conserved and unique elements present in the Bro1 domains of ALIX and BROX.