An acid-resistant peptide solubility tag (an “inclusion body tag”) is provided that is effective in producing peptides of interest in an insoluble form. Fusion peptide constructs comprising the inclusion body tag fused to a peptide of interest are provided. An acid cleavable peptide moiety separates the inclusion body tag from the peptide of interest so that acid hydrolysis can be used during subsequent processing steps to separate the tag from the desired peptide of interest. The present inclusion body tag's resistance to acid hydrolysis facilitates easier and cleaner separation of the peptide of interest after acid hydrolysis. Specifically, a ketosteroid isomerase-derived inclusion body tag is provided that has been engineered to be more resistant to acid hydrolysis.