Isolation of newly expressed surface T cell antigen receptor complexes by serial precipitation with anti-TCR antibodies and immobilized streptavidin

A useful radiolabeling method for detection of T cell receptor glycoproteins bearing immature and mature N-linked glycans

The antigen receptor expressed on most T lymphocytes, the T cell receptor (TCR), is a multisubunit complex containing TCR alpha,beta and CD3-gamma,delta, epsilon and zeta chains, in the form alphabetadeltaepsilongammaepsilonzetazeta. The intracellular transport of TCR proteins is routinely measured by the extent of processing of N-linked glycan chains present on TCR alpha,beta and CD3 gamma,delta subunits, which are converted from immature (high mannose) oligosaccharides in the endoplasmic reticulum (ER) to mature (complex type) species within the Golgi complex. In the current study, a radiolabeling method is described that is useful for the study of TCR glycoproteins bearing immature and mature N-linked oligosaccharide chains. Specifically, we report the use of [3H] galactose as a radioactive tracer of TCR proteins containing glucose and galactose residues, present in immature and mature N-linked oligosaccharides, respectively. This technique is rapid, simple, and sensitive, and provides a unique approach for studying the processing and subcellular localization of T cell receptor proteins.

Modification of the T cell antigen receptor (TCR) complex by UDP-glucose:glycoprotein glucosyltransferase. TCR folding is finalized convergent with formation of alpha beta delta epsilon gamma epsilon complexes

Most T lymphocytes express on their surfaces a multisubunit receptor complex, the T cell antigen receptor (TCR) containing alpha, beta, gamma, delta, epsilon, and zeta molecules, that has been widely studied as a model system for protein quality control. Although the parameters of TCR assembly are relatively well established, little information exists regarding the stage(s) of TCR oligomerization where folding of TCR proteins is completed. Here we evaluated the modification of TCR glycoproteins by the endoplasmic reticulum folding sensor enzyme UDP-glucose:glycoprotein glucosyltransferase (GT) as a unique and sensitive indicator of how TCR subunits assembled into multisubunit complexes are perceived by the endoplasmic reticulum quality control system. These results demonstrate that all TCR subunits containing N-glycans were modified by GT and that TCR proteins were differentially reglucosylated during their assembly with partner TCR chains. Importantly, these data show that GT modification of most TCR subunits persisted until assembly of CD3alpha beta chains and formation of CD3-associated, disulfide-linked alpha beta heterodimers. These studies provide a novel evaluation of the folding status of TCR glycoproteins during their assembly into multisubunit complexes and are consistent with the concept that TCR folding is finalized convergent with formation of alpha beta delta epsilon gamma epsilon complexes.

Multiple dimeric forms of human CD69 result from differential addition of N-glycans to typical (Asn-X-Ser/Thr) and atypical (Asn-X-cys) glycosylation motifs

CD69 is expressed on the surface of all hematopoietically derived leukocytes and is suggested to function as a multipurpose cell-surface trigger molecule important in the development and activation of many different cell types. Human CD69 contains only a single consensus sequence for N-linked oligosaccharide addition within its extracellular domain (Asn-Val-Thr), yet exists as two distinct glycoforms that are assembled together into disulfide-linked homodimers and heterodimers. The molecular basis for human CD69 heterogeneity has remained elusive. In the current report we show that human CD69 glycoforms are generated before the egress of CD69 proteins from the endoplasmic reticulum to the Golgi and are synthesized under conditions where Golgi processing is inhibited, effectively ruling out the possibility that CD69 heterogeneity results from the differential processing of a single glycosylation site in the Golgi complex. Importantly, these data demonstrate that contrary to current belief, not one but two sites for N-glycan addition exist within the human CD69 extracellular domain and identify the second, "cryptic" CD69 N-glycan attachment site as the atypical Cys-containing glycosylation motif, Asn-Ala-Cys. The results in this study provide a molecular basis for human CD69 heterogeneity and show that multiple dimeric forms of human CD69 result from the variable addition of N-glycans to atypical and typical glycosylation motifs within the CD69 extracellular domain.