Putative mechanisms of action of STMC6 (single-transmembrane proteins with conserved 6 cysteines, including Shisa) proteins. This diagram depicts four putative mechanisms of STMC6 proteins in regulating other transmembrane proteins. (a). The STMC6 protein can decrease cell surface expression of a target transmembrane protein by recruiting a NEDD4 family E3 ubiquitin ligase to ubiquitinate the target, possibly resulting in its degradation or relocalization. (b). The STMC6 protein can enhance cell surface expression of a target transmembrane protein by competitively interacting with the WW domains of a NEDD4 family E3 ubiquitin ligase and thus blocking the interaction between the E3 ligase and the target. (c). The STMC6 protein Shisa9 (CKAMP44) desensitizes the AMPA receptor by interacting with it. Shisa9 can also potentially interact with PDZ-domain-containing proteins via a C-terminal motif. (d). The STMC6 protein Shisa1 from Xenopus retains receptors in the ER and inhibits their cell surface expression.