Contains a GTPase activator for the Rho-type GTPases (RhoGAP) domain that would be able to negatively regulate the actin cytoskeleton as well as cell spreading. However, also contains N-terminally a BAR-domin which is able to play an autoinhibitory effect on this RhoGAP activity. Precursor of the histocompatibility antigen HA-1. More generally, minor histocompatibility antigens (mHags) refer to immunogenic peptide which, when complexed with MHC, can generate an immune response after recognition by specific T-cells. The peptides are derived from polymorphic intracellular proteins, which are cleaved by normal pathways of antigen processing. The binding of these peptides to MHC class I or class II molecules and its expression on the cell surface can stimulate T-cell responses and thereby trigger graft rejection or graft-versus-host disease (GVHD) after hematopoietic stem cell transplantation from HLA-identical sibling donor. GVHD is a frequent complication after bone marrow transplantation (BMT), due to mismatch of minor histocompatibility antigen in HLA-matched sibling marrow transplants. Specifically, mismatching for mHag HA-1 which is recognized as immunodominant, is shown to be associated with the development of severe GVHD after HLA-identical BMT. HA-1 is presented to the cell surface by MHC class I HLA-A*0201, but also by other HLA-A alleles. This complex specifically elicits donor-cytotoxic T-lymphocyte (CTL) reactivity against hematologic malignancies after treatment by HLA-identical allogenic BMT. It induces cell recognition and lysis by CTL. HA-1 forms a complex with MHC class I HLA-A*0201. Expressed on cells of the hematopoietic lineage. Detected in dendritic cells and epidermal Langerhans cells. Expressed in peripheral blood mononuclear cells, in all leukemia/lymphoma cell lines. Detected also in some solid tumors and tissues such as cancerous and non-cancerous tissue. Rho-GAP domain is able to regulate RhoGTPase activity, actin cytoskeleton and cell spreading. However N-terminally BAR domain plays an autoinhibitory role. The HA-1H allele is presented on the cell surface and recognized by CTL, whereas the HA-1R allele is poorly represented by HLA-A and non-immunogenic, although HA-1R allelic frequency is the highest (PubMed:9820595, PubMed:16399573). Infusion of lymphocyte from mHag HA-1-negative donors results in a durable remission in mHag HA-1-positive patients with leukemia or multiple myeloma. Extended N-terminus.