GTPase activating protein (GAP) which specifically converts GTP-bound RAC1 and CDC42 in their inactive GDP-bound form. The GAP activity is enhanced by the non-covalent binding of K-29 and K-48 polyubiquitin chains. Localization to membranes is increased by binding of poly-ubiquitin chains (PubMed:23223568). Enriched in tangle aggregates in cells of Alzheimer's disease brain (PubMed:23223568). Expressed in brain (at protein level). Based on a naturally occurring readthrough transcript which produces a SH3BP1-PDXP fusion protein. Translation initiation occurs at a non-canonical CUG codon. Produced by alternative initiation at Met-73 of isoform long BGIN. Translation initiation occurs at a non-canonical CUG codon. Truncated N-terminus. Unusual initiator. The initiator methionine is coded by a non-canonical CTG leucine codon at position 73.