Expression and localization of α-adaptin isoforms

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Abstract
There are two α-adaptin genes, αA and αC, which in brain encode proteins of of Mr 108×103 and 104×103, respectively. Although both mRNAs can be detected on northern blots of brain and liver, the higher molecular mass polypeptide can only be detected on western blots of brain. Here we explain these observations by showing that αA is alternatively spliced and that the protein product in most tissues is different from the one expressed in brain in that it is missing 21 amino acids within the hinge region, giving it a similar mobility to that of αC. Monospecific antibodies were raised against the various α-adaptin isoforms and used to compare their distribution in cells and tissues. Both αA and αC are co-assembled into the same coated pits, and the larger isoform of αA is co-assembled with the smaller isoforms of α-adaptin, both in cells that naturally express it and in transfected cells. Examination of brain and spinal cord sections, labelled either for the larger isoform of αA or for αC, reveals that that the two are to some extent differentially distributed, consistent with previous in situ hybridisation studies. This finding, combined with the observation that there is considerable variability in the relative expression of the two isoforms in different tissues, indicates that the two genes are switched on in response to different stimuli. Moreover, the larger isoform of αA appears to be more efficiently concentrated in the nerve terminals than αC, which is found not only at the terminals but also diffusely distributed in the cell bodies and dendrites. This suggests that αC may play more of a role in the recycling of membrane components throughout the cell.