Proposed model for the function of the MIA complex in regulation of I1 dynein. (A) In wild type, the MIA complex is localized in a position to mediate signals to I1 dynein motor complex (orange). Kinases and phosphatases (white in A and red in B), such as CK1, PKA, and PP2A, are predicted to be positioned at the base of the I1 dynein complex, in position to regulate phosphorylation of the IC138 substrate of I1 dynein. (B) In the absence of the MIA complex, these signaling proteins (red) are present but may be mispositioned relative to IC138 and I1 dynein, which are flexible, resulting in hyperphosphorylated IC138 in the mia mutants. KIP, kinase–phosphatase complex; p, phosphorylation.