The deduced protein sequence of the human carboxypeptidase N high molecular weight subunit reveals the presence of leucine-rich tandem repeats

Human plasma carboxypeptidase N is a 280-kDa tetramer with two high molecular mass (83-kDa) glycosylated subunits which protect the two 50-kDa catalytic subunits and keep them in the circulation. An initial clone for the 83-kDa subunit was obtained by screening two lambda gt11 human liver cDNA expression libraries with antiserum specific for carboxypeptidase N or the 83-kDa subunit. The libraries were rescreened with the labeled cloned cDNA, and the largest clone obtained (2536-base pair insert) was completely sequenced. The deduced protein sequence matched the sequence of several tryptic peptides from the 83-kDa subunit but did not contain the NH2-terminal sequence. The remaining portion of the protein coding sequence was synthesized by the polymerase chain reaction, cloned, and sequenced. The composite cDNA sequence is 2870 base pairs long with an open reading frame of 1608 base pair coding for a protein of 536 amino acids (Mr = 58,762). The protein sequence contains seven potential N-linked glycosylation sites and a threonine/serine-rich region which is a potential site for attachment of O-linked carbohydrate. The most striking feature is a region (residues 68-355) containing 12 leucine-rich tandem repeats of 24 residues with the following consensus sequence: P-X-X-alpha-F-X-X-L-X-X-L-X-X-L-X-L-X-X-N-X-L-X-X-L (X = any amino acid and alpha = aliphatic amino acids, I, L, or V). This repeating pattern is found in the leucine-rich alpha 2-glycoprotein and in other proteins where it might mediate interactions with macromolecules. This region also contains five sequences with heptad repeating leucine residues comprising a leucine zipper motif. The leucine-rich domain likely constitutes an important structural or functional element in the interactions of the 83- and 50-kDa subunits to form the active tetramer of carboxypeptidase N.