The
agyria (
lissencephaly)/
pachygyria phenotypes are catastrophic developmental diseases characterized by abnormal folds on the surface of the brain and disorganized cortical layering. In addition to mutations in at least four genes--LIS1, DCX, ARX and RELN--mutations in a human
alpha-tubulin gene, TUBA1A, have recently been identified that cause these diseases. Here, we show that one such mutation, R264C, leads to a diminished capacity of de novo
tubulin heterodimer formation. We identify the mechanisms that contribute to this defect. First, there is a reduced efficiency whereby quasinative
alpha-tubulin folding intermediates are generated via
ATP-dependent interaction with the
cytosolic chaperonin CCT. Second, there is a failure of CCT-generated folding intermediates to stably interact with TBCB, one of the five
tubulin chaperones (TBCA-E) that participate in the pathway leading to the de novo assembly of the
tubulin heterodimer. We describe the behavior of the R264C mutation in terms of its effect on the structural integrity of
alpha-tubulin and its interaction with TBCB. In spite of its compromised folding efficiency, R264C molecules that do productively assemble into heterodimers are capable of copolymerizing into dynamic microtubules in vivo. The diminished production of TUBA1A
tubulin in R264C individuals is consistent with haploinsufficiency as a cause of the disease phenotype.