Rotational and lateral dynamics of I-Ak molecules expressing cytoplasmic truncations

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Abstract
Rotational and lateral diffusion of I-Ak molecules with various α and β chain cytoplasmic truncations known to affect class II function were measured to assess the role of cytoplasmic domains in regulating I-Ak molecular motions. Deletion of all 12 α chain C-terminal residues and all 18 corresponding β chain residues (α-12/β-18) is known to abrogate translocation of protein kinase C to the nucleus upon class II cross-linking. Similarly, truncation of the entire cytoplasmic α chain domain and the 10 C-terminal residues of the β chain impairs presentation of antigenic peptides to T cells. The rotational correlation time of the wild-type molecule, 11.9 ± 2.6 μs as measured by time-resolved phosphorescence anisotropy, decreased to 7.2 ± 3.7 μs in the fully truncated α-12/β-18 protein. Other truncated class II molecules exhibited only small changes in molecular rotation rates relative to the wild-type. The rate of lateral diffusion of the fully truncated molecule, measured with two independent methods, 2.3 × 10–10 cm2/s, was comparable with that of the wild-type molecule. Thus, it appears that the α and β chain cytoplasmic domains regulate the molecular motions of unperturbed I-Ak molecules only modestly, despite the known involvement of these regions in class II signaling. Various explanations for this behavior are discussed, e.g. the possibility that class II membrane complexes are sufficiently large that association and dissociation of specific signaling proteins during antigen presentation do not significantly perturb the apparent molecular motions of the complex.