β subunits of voltage-gated calcium channels (VGCCs) regulate channel trafficking and function, thereby shaping the intensity and duration of intracellular changes in calcium. β subunits share limited sequence homology with the Src homology 3-guanylate kinase (SH3-GK) module of membrane-associated guanylate kinases (MAGUKs). Here, we show biochemical similarities between β subunits and MAGUKs, revealing important aspects of β subunit structure and function. Similar to MAGUKs, an SH3-GK interaction within β subunits can occur both intramolecularly and intermolecularly. Mutations that disrupt the SH3-GK interaction in β subunits alter channel inactivation and can inhibit binding between the α1 and β subunits. Coexpression of β subunits with complementary mutations in their SH3 and GK domains rescues these deficits through intermolecular β subunit assembly. In MAGUKs, the SH3-GK module controls protein scaffolding. In β subunits, this module regulates the inactivation of VGCCs and provides an additional mechanism for tuning calcium responsiveness.
