The lipid-binding PX domain of RRC-1 (ARHGAP32/33) is required for optimal assembly and function of integrin adhesion complexes at the muscle cell boundary in C. elegans.

Integrin adhesion complexes (IACs) are a network of many proteins that serve as anchors of the cell to the extracellular matrix (ECM). In muscle, IACs located at costameres also serve to transmit the force of muscle contraction to the outside of the cell. We have reported that IACs, which are found at the bases of dense bodies and M-lines, and at muscle cell boundaries (MCB) in Caenorhabditis elegans muscle, require the RacGEF PIX-1 for their proper assembly or maintenance. We have reported that a RacGAP for the PIX pathway is RRC-1, which is in a complex with PIX-1, and that RRC-1 is required for assembly or maintenance of IACs at MCBs. Our previous studies suggested that RRC-1 might be associated with the muscle cell membrane, and here we present evidence that this occurs via its PX domain, a domain that is known to bind to membrane phosphoinositides (PIPs). We predict the existence of a PX domain based on bioinformatic analysis and AlphaFold3, which includes conserved residues characteristic of most PX domains and a PIP-binding site. This region of RRC-1 binds to phosphoinositides in vitro. Analysis of a nematode strain that has an in-frame deletion of the PX domain generated by CRISPR/Cas9 indicates that normal localization of RRC-1 to the MCB requires both its PX domain and the PIX scaffold protein GIT-1. Lastly, overexpression experiments suggest that both the PX domain and the RhoGAP domain of RRC-1 each contribute to the proper function of RRC-1 at the MCB. Our study highlights the importance of RRC-1's lipid interactions at the cell membrane for the proper assembly and function of IACs in C. elegans muscle.