Purification of Golgi adaptor protein 1 from bovine adrenal gland and characterization of its β1 (β′) subunit by microsequencing

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Abstract
A method for the purification of the Golgi adaptor protein 1 from bovine adrenal gland tissue was devised to investigate the relationship of its β1 (formerly referred to as β′) subunit to known β‐type sequences. Adrenal gland tissue was chosen for this study because it yielded 2–3 times more adaptor protein 1 than a comparable preparation from bovine brain. Like its neuronal isoform, the β1 subunit from adrenal gland adaptor protein 1 is readily cleaved by trypsin into a 63‐kDa N‐terminal fragment and a 40‐kDa C‐terminal fragment, while the γ subunit is largely refractory to digestion. Based on microsequencing of 167 residues from the 63‐kDa fragment, we noted 11 differences to the corresponding region of the β2 (formerly β) subunit of the plasma membrane adaptor protein 2, but only one difference to the corresponding region of a β‐type protein encoded by the rat cDNA clone AP105a which is supposed to be a variant of the β2 subunit of the plasma membrane adaptor protein 2 [Kirchhausen, T., Nathanson, K. L., Matsui, W., Vaisberg, A., Chow, E. P., Burne, C., Keen, J. H. & Davis, A. E. (1989) Proc. Natl Acad. Sci. USA 84, 8805–8809]. Alignment of 187 residues from the 40‐kDa β1 C‐terminal fragment revealed differences in 77 positions to the corresponding region of the β2 subunit and differences in 23 positions compared to the supposed β2‐like protein. These findings suggest that the protein encoded by the rat cDNA clone AP105a is more closely related to the β1 subunit of the bovine adrenal Golgi adaptor protein 1 than to the β2 subunit of the rat plasma membrane adaptor protein 2.