p24 and p26, structurally related cell surface molecules identified by monoclonal antibody BA-2

CD81 (TAPA-1): a molecule involved in signal transduction and cell adhesion in the immune system

CD81 (TAPA-1) is a widely expressed cell-surface protein involved in an astonishing variety of biologic responses. It has been cloned independently several times for different functional effects and is reported to influence adhesion, morphology, activation, proliferation, and differentiation of B, T, and other cells. On B cells CD81 is part of a complex with CD21, CD19, and Leu13. This complex reduces the threshold for B cell activation via the B cell receptor by bridging Ag specific recognition and CD21-mediated complement recognition. Similarly on T cells CD81 associates with CD4 and CD8 and provides a costimulatory signal with CD3. In fetal thymic organ culture, mAb to CD81 block maturation of CD4-CD8- thymocytes, and expression of CD81 on CHO cells endows those cells with the ability to support T cell maturation. However, CD81-deficient mice express normal numbers and subsets of T cells. These mice do exhibit diminished antibody responses to protein antigens. CD81 is also physically and functionally associated with several integrins. Anti-CD81 can activate integrin alpha 4 beta 1 (VLA-4) on B cells, facilitating their adhesion to tonsilar interfollicular stroma. Similarly, anti-CD81 can activate alpha L beta 2 (LFA-1) on human thymocytes. CD81 can also affect cognate B-T cell interactions because anti-CD81 increases IL-4 synthesis by T cells responding to antigen presented by B cells but not by monocytes. The tetraspanin superfamily (or TM4SF) includes CD81, CD9, CD37, CD53, CD63, CD82, CD151, and an increasing number of additional proteins. Like CD81, several tetraspanins are involved in cell adhesion, motility, and metastasis, as well as cell activation and signal transduction.

CD9, but not other tetraspans, associates with the beta1 integrin precursor

The molecules of the tetraspan superfamily have two unequal extracellular domains separated by four transmembrane (TM) domains. These molecules are associated on the cell surface with each other and with other partner molecules, in particular beta1 integrins. We now show that CD9 associates with the precursor of the beta1 integrin (pre beta1). This association is detected as early as 15 min after metabolic labeling, and the use of Brefeldin A demonstrates that it does not require Golgi modifications of either CD9 or integrin beta1. The specificity of this interaction is demonstrated by the fact that other tetraspans, CD63, CD81, and CD82, do not associate with pre beta1, and CD9 does not associate with immature human histocompatibility leukocyte antigen class I. In order to localize the region of CD9 responsible for the association with the beta1 integrin, we have generated two reciprocal chimeric CD9/CD82 molecules with the junction localized just after the third TM region. The large extracellular loop of CD9 or the fourth TM domain, or both, appear to be sufficient to mediate an association with the mature integrin with a high efficiency, compared to CD82. By contrast, association with pre beta1 requires at least two regions of the molecule. Mutation of CD9 at the consensus site of the tetraspan superfamily, localized between the second and the third TM domain, did not impair the co-precipitation of pre beta1. Finally, because pre beta1 associates with calnexin, we have investigated a possible association of CD9 with this chaperone molecule. CD9 associates with calnexin independently of its association with the beta1 integrin, suggesting that calnexin could be involved in the processing of CD9.

Myristic acid is incorporated into the two acylatable domains of the functional glycoprotein CD9 in ester, but not in amide bonds

CD9 is a signal-initiating glycoprotein of uncertain membrane insertion which contains more than one locus of acylation and is distinguished by being the major acylatable platelet protein. The N-terminus of CD9 is blocked to Edman degradation. We investigated whether [3H]myristic acid could be incorporated into CD9, whether that incorporation occurred via an amide linkage, and whether myristate and palmitate were differentially incorporated into the two domains. Pulse-labeling studies, performed on the human osteogenic sarcoma cell line SKOSC which expresses 22 and 24 kDa variants of CD9 demonstrated that the respective precursors of 20.5 and 23 kDa were not radiolabeled by either [3H]myristic acid or [3H]palmitic acid, but that both fatty acids could be ligated to CD9 during the later stages of protein maturation. The failure to incorporate myristic acid cotranslationally suggest that CD9 does not contain amino-terminal amide-bonded myristic acid. Incorporation of radiolabel from both fatty acids proceeded very rapidly and could be visualized after a 10 s pulse. Although myristic acid was partially metabolized into palmitic acid, incorporation of authentic [3H]myristate into CD9 could be demonstrated. The myristic acid bonds were shown to be as sensitive to hydroxylamine treatment as those linking palmitate. Both fatty acids were also incorporated into CD9 in hydroxylamine-sensitive bonds in the presence of cycloheximide, reaching 30-40% of the levels in untreated controls. The sensitivity of myristate ligands to hydroxylamine demonstrates that this fatty acid is not linked via amide, but rather via ester bonds. The sensitivity of [3H]myristate and [3H]palmitate bonds to 2-mercaptoethanol further suggests that either fatty acid is linked via thioester rather than hydroxyester bonds to each domain on CD9. Limited proteolysis analysis with Staphylococcus aureus V8 proteinase of CD9, labeled in the absence or presence of cycloheximide, showed that [3H]myristic acid and [3H]palmitic acid labeled identical peptides, and to the same extent, suggesting that myristate is an alternative substrate for the transacylase(s) involved.

The functional glycoprotein CD9 is variably acylated: localization of the variably acylated region to a membrane-associated peptide containing the binding site for the agonistic monoclonal antibody 50H.19

Recent studies have shown that [3H]palmitic acid strongly labels both glycosylated forms (gp22 and gp24) of the signal-initiating cell surface glycoprotein CD9. We performed a two-dimensional limited proteolysis analysis with Staphylococcus aureus V8 proteinase in order to localize the palmitylation sites to final peptides on both glycosylated forms of CD9. Analysis of [3H]leucine- and [3H]amino acid mixture-labeled gp22 delineated 4 final peptides of 11, 8, 7 and 4 kDa. gp24 produced a similar pattern with the exception that the 11 kDa peptide was replaced by an N-glycosylated 13 kDa peptide. Since all four final peptides (total molecular mass of 30/32 kDa) could not be accommodated by a parent molecule of 22/24 kDa, it is likely that one of the final peptide coexists in two differently modified states. Palmitic acid labeled the 11 kDa/13 kDa final peptides, and the 7 kDa final peptide, with equal intensity, but was not incorporated into the 4 kDa final peptide, demonstrating that fatty acid is ligated in two distinct regions of the molecule. The 8 kDa final peptide was strongly labeled by [3H]palmitic acid, but only weakly by [3H]leucine. We present evidence that this peptide is derived by further acylation of the region defined by the 7 kDa peptide, and that this occurs in only 15% of the molecules. Palmitic acid is turned over faster at these additional sites, indicating that they may be more accessible to membrane transacylases. Proteolysis of CD9 on the intact cell with papain enabled the highly acylated region to be localized to a membrane-associated fragment which contains the binding site for the agonistic monoclonal antibody 50H.19. The co-localization of a functional domain with a region of variable acylation suggests that acylation events may play a role in the transduction of the signal initiated by interaction of the antibody with CD9.

Biochemical properties of a novel rabbit thymocyte specific class I-like antigen

Monoclonal antibody 5E2 identifies a new rabbit thymocyte specific cell surface molecule designated R-Ta. SDS-PAGE of molecules immunoprecipitated by 5E2 shows that R-Ta exists as a non-covalently associated hetero-dimer consisting of a light polypeptide chain (mol. wt approximately 12,000) and a bi-molecular species of a heavy chain (mol. wts of 45,000 and 40,000). The difference between the two forms of heavy chain can be attributed to different degrees of glycosylation. Each form of the R-Ta heavy chain has a polypeptide mol. wt of 34,000. At least three N-linked oligosaccharides and no significant O-linked sugars were found associated with R-Ta. Two dimensional electrophoresis of V8 protease peptide maps also indicate that the two forms of the heavy chains are similar, if not identical, in polypeptide primary structure. The light polypeptide was found to be serologically and structurally identical to beta-2-microglobulin. This was demonstrated in a previous study by reaction with goat anti-beta-2-microglobulin antisera. In this investigation the structural identity with beta-2-microglobulin was demonstrated by partial amino terminal sequence analysis. The partial amino acid sequence for 18 steps of the R-Ta heavy chain was also determined. A comparison of the amino acid sequence with other known sequences for the conventional Class I molecules of man, mouse and rabbit did not reveal any homology. Thus R-Ta is a new T-cell surface protein, and like human CD1, carries the unique distinction of thymocyte specificity, is beta-2-microglobulin associated, but is not Class I related.

The functional cell surface glycoprotein CD9 is distinguished by being the major fatty acid acylated and a major iodinated cell-surface component of the human platelet

We showed that a 22 kDa protein (which comigrated with the leukocyte differentiation antigen CD9 as determined by immunoblotting with the platelet-activating mAb 50H.19) is a major iodinated component of the platelet surface. The iodinated protein was identified as CD9 by limited proteolysis analysis. The major acylated protein in platelets incubated with [3H]palmitic acid also had a mobility of 22 kDa. The radiolabelled fatty acid in CD9 appears to be ester bonded, as it is removed by treatment with hydroxylamine. Non-enzymatic ligation of the fatty acid is not involved. Since platelets lack protein synthetic capacity, the palmitolation of a surface protein indicates the existence of a plasma-membrane located transacylase which functions independently of protein synthesis. Limited proteolysis analysis of the palmitylated protein obtained by immunoprecipitation with mAb 50H.19 confirmed its identity as CD9. An additional novel minor component of 27 kDa was detected in platelets by immunoprecipitation of 125I-surface-labelled, or [3H]palmitic acid-labelled protein, and by immunoblotting with mAb 50H.19. The analogous cleavage patterns obtained by the limited proteolysis analysis of the 22, 24 and 27 kDa glycoproteins suggest that they may be differently modified variants of a single polypeptide.