PSD-95 and SAP97 exhibit distinct mechanisms for regulating K+ channel surface expression and clustering

Mechanisms of ion channel clustering by cytoplasmic membrane-associated gua nylate kinases such as postsynaptic density 95 (PSD-95) and synapse-associa ted protein 97 (SAP97) are poorly understood. Here, we investigated the int eraction of PSD-95 and SAP97 with voltage-gated or Kv K+ channels. Using Kv channels with different surface expression properties, we found that clust ering by PSD-95 depended on channel cell surface : expression. Moreover, PS D-95-induced clusters of Kv1 K+ channels were present on the cell surface. This was most dramatically demonstrated for Kv1.2 K+ channels, where surfac e expression and clustering by PSD-95 were coincidentally promoted by coexp ression with cytoplasmic Kv beta subunits, Consistent with a mechanism of p lasma membrane channel-PSD-95 binding, coexpression with PSD-95 did not aff ect the intrinsic surface expression characteristics of the differ ent Ky c hannels. In contrast, the interaction of Kv1 channels with SAP97 was indepe ndent of Kv1 surface expression, occurred intracellularly, and prevented fu rther biosynthetic trafficking of Kv1 channels. As such, SAP97 binding caus ed an intracellular accumulation of each Kv1 channel tested, through the ac cretion of SAP97 channel clusters in large (3-5 mu m) ER-derived intracellu lar membrane vesicles. Together, these data show that ion channel clusterin g by PSD-95 and SAP97 occurs by distinct mechanisms, and suggests that thes e channel-clustering proteins may play diverse roles in regulating the abun dance and distribution of channels at synapses and other neuronal membrane specializations.