MPP4 (MAGUK p55 scaffold protein 4) is a retina-specific member of the membrane-associated guanylate kinase (MAGUK) protein family that functions as a molecular scaffold at specialized membrane-cytoskeleton interfaces 1. The protein contains characteristic MAGUK motifs including an N-terminal PDZ domain, SH3 region, and C-terminal guanylate kinase-like domain, placing it in the p55 subfamily 1. MPP4 is abundantly expressed in photoreceptor cells and localizes specifically to connecting cilia and synaptic terminals of both cone and rod photoreceptors, as well as postsynaptic structures in the outer plexiform layer 23. The protein functions by organizing multiprotein complexes at photoreceptor ribbon synapses, interacting with adaptor proteins including Veli3 and Psd95-beta through L27 domain interactions 45. Additional associated proteins include recoverin and Hsc70, suggesting roles in calcium signaling and protein folding at synapses 4. MPP4's location on chromosome 2-33 coincides with the RP26 locus for autosomal recessive retinitis pigmentosa, though mutation screening has not established it as the causative gene 12. The protein's specific localization and scaffolding function suggest it plays a critical role in photoreceptor synapse organization and visual signal transmission.