The association of MCL and FcεRI-γ is surprising given that MCL lacks the canonical motif — a positively charged amino acid in the transmembrane
domain — for binding activating adaptors, and others have tried and failed to demonstrate this association . The Thr38 residue of MCL that they postulate mediates the association with FcεRI-γ is conserved in the rat, but we have been unable to demonstrate any direct association of rat MCL to FcεRI-γ. The direct recognition of TDM that Miyake et al.  describe suggests that MCL can play a role in TDM recognition independently of its association with Mincle. In our hands, rat MCL reporters are not stimulated by mycobacteria, while Mincle reporters are stimulated by mycobacteria (Supporting find more Information Fig. 1). Although it is unknown SB203580 cell line exactly how TDM is recognized by Mincle, both TDM and the Malassezia ligand for Mincle  are glycolipids. Although the presence of both the saccharide and lipid portions of TDM is important for recognition by Mincle , it is likely that the sugar moiety is the major antigen determinant. Sugar recognition is mediated by the lectin domain, and within this domain,
a tripeptide motif is thought to heavily influence the type of sugar moieties that can be recognized. An EPX motif (where X is usually asparagine) mediates binding to glucose moieties such as found in TDM . The EPN tripeptide motif is conserved in Mincle from rat, mouse, and human, and Mincle from all three species is able to mediate recognition of Malassezia and mycobacterial cord factor ([8, 10, 11] and our unpublished data). For MCL, the EPX motif is conserved in rat and human (although X is D in human and K in rat), but in mouse only the E is conserved. This suggests that there is little selection pressure on this motif in MCL or that different ligands are recognized by the different species. In addition, MCL has previously been shown to have very weak sugar binding . One possible explanation for the differences we
see is that MCL binds rather to the lipid Morin Hydrate portion. Although lipid binding by C-type lectins is unusual, it is not unheard of — surfactant proteins A and D are both able to bind to a range of lipids via their carbohydrate recognition domains . In their experimental system with purified TDM, the lipid portion is presumably exposed and available for binding to MCL reporter lines; in our system with intact mycobacteria, the lipid portion may be buried in the membrane and thus unable to stimulate our MCL reporters. If this hypothesis is correct, the Mincle/MCL heterodimer described here could allow co-ordinate binding to the TDM molecule, with Mincle binding to the sugar moiety and MCL to the lipid. The congenic rat strains DA.APLEC (APLEC gene complex from PVG)  and DA.NKCB (NK complex from PVG)  were maintained under conventional conditions.