Lymphocyte surface receptor that binds CAML, nucleic acids encoding the same and methods of use thereof

An isolated polypeptide encoded by a nucleic acid consisting of at least 18 consecutive nucleotides of a nucleotide sequence, wherein said nucleotide sequence encodes a Transmembrane Activator and CAML Interactor (TACI) protein having the amino acid sequence of SEQ ID NO:2;

• wherein the polypeptide functions as a cell surface signaling protein comprising an extracellular domain, a membrane spanning segment, and a cytoplasmic domain that has a binding affinity for CAML;
• and wherein said TACI protein is a plasma membrane receptor that interacts directly with an intracellular organelle in lymphocytes.

A proteolytic fragment of an isolated Transmembrane Activator and CAML Interactor (TACI) protein;

• wherein said TACI protein functions as a cell surface signaling protein comprising an extracellular domain, a membrane spanning segment, and a cytoplasmic domain that has a binding affinity for CAML, an wherein said TACI protein is a plasma membrane receptor that interacts directly with an intracellular organelle in lymphocytes.

An isolated polypeptide comprising an N-terminal fragment of a Transmembrane Activator and CAML Interactor (TACI) protein;

• wherein said TACI protein functions as a cell surface signaling protein comprising an extracellular domain, a membrane spanning segment, and a cytoplasmic domain that has a binding affinity for CAML, wherein said TACI protein is a plasma membrane receptor that interacts directly with an intracellular organelle in lymphocytes;
• and
• wherein said N-terminal fragment corresponds to the regulatory extracellular domain of the TACI.

An isolated polypeptide comprising a C-terminal fragment of a Transmembrane Activator and CAML Interactor (TACI) protein;

• wherein said TACI protein functions as a cell surface signaling protein comprising an extracellular domain, a membrane spanning segment, and a cytoplasmic domain that has a binding affinity for CAML, wherein said TACI protein is a plasma membrane receptor that interacts directly with an intracellular organelle in lymphocytes;
• and
• wherein said C-terminal fragment is sufficient to bind the N-terminal 146 amino acids of CAML.