MHCI and insulin receptors coimmunoprecipitate from WT hippocampal lysates, and insulin receptor expression and trafficking is normal in MHCI-deficient mice. A, Western blots of whole-brain lysates from adult (P30) WT or NIRKO mice probed for IRβ using clone C19. The loss of insulin receptor labeling (IRβ; ∼95 kDa) in NIRKO brain samples as well as the absence of IGF-1 receptor labeling (∼115 kDa) in either genotype demonstrate the specificity of clone C19 for insulin receptors. Bottom, The same samples stripped and reprobed for the abundant intracellular protein GAPDH. B, Western blotting for MHCI and IRβ in hippocampal lysates or IRβ immunoprecipitates. IRβ antibodies selectively coprecipitate MHCI, but not other cell surface (GluR1) or intracellular (GAPDH) proteins. IP:IgG, Isotype control immunoprecipitation. C, D, Western blotting of WT and β2m^−/−TAP^−/− hippocampal lysates probed for IRβ (C19) or IRα (N20) subunits (C), and hippocampal cell surface fractions probed for IRβ (C19; D). Total and surface levels of insulin receptor protein are indistinguishable between genotypes. Insulin receptor values are normalized to GAPDH and represented as the percentage of WT (mean ± SEM). Total IRβ: WT, 100 ± 2.3%; β2m^−/−TAP^−/−, 111.6 ± 5.5%; n = 6 animals. Total IRα: WT, 102 ± 1.3%; β2m^−/−TAP^−/−, 99.1 ± 4.5%; n = 3. Surface: WT, 100 ± 2.0%; β2m^−/−TAP^−/−, 101 ± 12.4%; n = 3. E, Western blots of total (S1) and synaptic (P3) fractions from hippocampus probed for IRβ (C19) and the presynaptic protein synaptophysin (Syn). Synaptic levels of insulin receptor protein are indistinguishable between genotypes. S1: WT, 100 ± 13.0%; β2m^−/−TAP^−/−, 100 ± 3.0%; n = 6 animals; P3: WT, 100 ± 5.0%; β2m^−/−TAP^−/−, 108 ± 4.0%; n = 6.