Interaction of carboxyl-terminal peptides of cytosolic-tail of apactin with PDZ domains of NHERF/EBP50 and PDZK-1/CAP70.

The C-terminal PDZ-binding motifs are required for polarized apical/basolateral localization of many membrane proteins. Ezrin-radixin-moesin (ERM) proteins regulate the organization and function of specific cortical structures in polarized epithelial cells by connecting filamentous (F)-actin to plasma membrane proteins through EBP50. Previous work showed that the membrane phosphoprotein apactin (an 80-kDa type I membrane protein derived from pro-Muclin) is associated with the acinar cell apical actin cytoskeleton and that this association is modulated by changes in the phosphorylation state of the apactin cytosolic tail. The carboxyl-terminal amino acids of apactin (-STKL-COOH) are predicted to form a type I PDZ-binding domain, similar to that of CFTR (-DTRL-COOH). Pairwise sequence comparison between NHERF/EBP50 and PDZK1/CAP70 PDZ domains reveals significant identity among the 83 amino-acid residues (12-92) of EBP50 and CAP70 (241-323), which are involved in the interaction with the carboxyl-terminal peptides (STKL-COOH and phosphomimetics) of apactin. Hence, the specificity and affinity of interactions are identical between them, which is corroborated with the two hybrid results. Substitution of all the four-carboxyl-terminal amino acids in the wild type to Ala reduces the interaction. Only the carbonyl oxygen and amide nitrogen of Ala are found to be involved in hydrogen bonding. Further, truncation of the wild carboxyl-terminal peptide to RGQPP-COOH, showed very low affinity of interaction with PDZ1 domain. Only the atom O(epsilon1) of Gln-2 hydrogen bonds with N(epsilon2) of His72 of PDZ domain. Ser-3 amino acid in wild type apactin protein (STKL-COOH) is not involved in hydrogen bonding with PDZ1 domain. However, substitution of Ser-3 to Asp-3 in PDTKL-COOH peptide increases the affinity of interaction of PDTKL-COOH with PDZ1 domain. Thus, carboxyl-terminal Asp(D) -3, Thr(T) -2, Lys(K) -1 and Leu(L) 0 are involved in numerous interactions with PDZ1 domains of NHERF/EBP50 and PDZK1/CAP70.