Data for: The CD4 transmembrane GGXXG and juxtamembrane (C/F)CV+C motifs mediate pMHCII-specific signaling independently of CD4-Lck interactions

We previously reported that the transmembrane GGXXG motif co-arose with
the cytoplasmic juxtamembrane (C/F)CV+C motif in eutherian (placental
mammal) CD4 and that constituent residues therein evolve under purifying
selection (Lee, et al., 2022). These data led us to discover that mutating
these motifs together increased CD4-Lck association but reduced CD3z,
Zap70, and Plcg1 phosphorylation levels as well as IL-2 production.
Because these mutants preferentially localized CD4-Lck pairs to
non-membrane raft fractions, one potential explanation for our results was
that they impaired proximal signaling by sequestering Lck away from
TCR-CD3. An alternative hypothesis is that the mutations directly impacted
TCR-CD3 signaling because the wild type motifs play a Lck-independent role
in signaling. Here we used a reductionist approach to discriminate between
these possibilities (Lee, et al., 2023). Our current data lead us to
conclude that: intracellular CD4-Lck interactions are not necessary for
pMHCII-specific signal initiation; the GGXXG and (C/F)CV+C motifs are key
determinants of CD4-mediated pMHCII-specific signal amplification; the
GGXXG and (C/F)CV+C motifs exert their functions independently of direct
CD4-Lck association. These data provide a mechanistic explanation for why
residues within these motifs are under purifying selection. The results
are also important to consider for biomimetic engineering of synthetic
receptors.