Plants have isoforms for acyl carrier protein that are expressed differently in different tissues

Two closely related isoforms of acyl carrier protein (I and II) have been purified from spinach leaves. Differences in the N-terminal amino acid sequence and amino acid composition indicate that these proteins are coded by different genes. The two spinach leaf isoforms have been resolved and characterized by ion-exchange high-performance liquid chromatography, by thin layer isoelectric focusing, and by differences in mobility upon polyacrylamide gel electrophoresis. Both isoforms are effectively bound by antibodies raised to acyl carrier protein I. However, in competition experiments isoform II is only about 40% effective in blocking isoform I binding to antibody. Therefore, the isoforms are immunologically related but hold only some antigenic sites in common. Immunoblot analysis ("Western blotting") of crude spinach leaf tissue extracts probed with antibody to acyl carrier protein I reveals both isoforms. In addition, both forms of acyl carrier protein are present in dark-grown leaf tissue and in isolated chloroplasts. However, in spinach seeds and roots only acyl carrier protein II can be detected. Similar results are observed with extracts of castor oil plant leaf and seed. Therefore, the expression of the two acyl carrier protein isoforms is tissue specific.