Acts as a regulatory protein that associates with GUCY2C and negatively modulates its heat-stable enterotoxin-mediated activation (By similarity). Stimulates SLC9A3 activity in the presence of elevated calcium ions (By similarity). Interacts with the C-terminal region of GUCY2C (By similarity). Interacts with C-terminal region of SLC9A3 and the interactions decrease in response to elevated calcium ion levels (By similarity). Interacts with the C-terminal region of SLC34A1 (PubMed:11099500). Interacts with USP2 isoform 2 (PubMed:26756164). In kidney, predominantly localized to the subapical compartment of proximal tubular cells (PubMed:11099500). In intestinal epithelial cells, localizes to the ileal brush border and in a juxtanuclear compartment (PubMed:19088451). Expressed in the membrane, with relatively low expression in the cytoplasm (PubMed:26756164). Expressed in kidney and small intestine. Not detected in heart, brain, spleen, lung, liver, skeletal muscle or testis. Isoform 2: Expressed in a circadian manner in the intestine.