Expression of the protein-tyrosine kinase p56lck by the pTRX vector yields a highly soluble protein recovered by mild sonication.

We have expressed the protein-tyrosine kinase p56lck as a fusion with glutathione-S-transferase (GST) or thioredoxin (TRX). While the GST-Lck fusion protein was found to be poorly soluble and solubilization techniques led to a high degree of degradation, the TRX-Lck fusion protein was found to be highly soluble. This solubilization was achieved by mild sonication in a simple low ionic strength buffer which makes the fusion protein immediately available for further use. Even in the absence of protease inhibitors, the TRX-Lck fusion protein exhibited no trace of degradation. We produced 40-60 mg of TRX-Lck/l of culture which was efficiently cleaved at the endopeptidase site. The TRX-Lck and GST-Lck fusion proteins exhibited similar phosphorylation activity.