The BAR domain of Arfaptin-like proteins, also called the Arfaptin domain, is a dimerization and lipid binding module that can detect and drive membrane curvature. Arfaptins are ubiquitously expressed proteins implicated in mediating cross-talk between Rac, a member of the Rho family GTPases, and Arf (ADP-ribosylation factor) small GTPases. Arfaptins bind to GTP-bound Arf1, Arf5, and Arf6, with strongest binding to GTP-Arf1. Arfaptins also bind to Rac-GTP and Rac-GDP with similar affinities. The Arfs are thought to bind to the same surface as Rac, and their binding is mutually exclusive. Mammals contain at least two isoforms of Arfaptin. Arfaptin 1 has been shown to inhibit the activation of Arf-dependent phospholipase D (PLD) and the secretion of matrix metalloproteinase-9 (MMP-9), an enzyme implicated in cancer invasiveness and metastasis. Arfaptin 2 regulates the aggregation of the protein huntingtin, which is implicated in Huntington disease. Arfaptins are single-domain proteins with a BAR-like structure. BAR domains form dimers that bind to membranes, induce membrane bending and curvature, and may also be involved in protein-protein interactions.
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