A recombinant expression system for the expression of a poly amino acid, peptide or protein is provided. The poly amino acid of interest is expressed as a fusion protein that includes an amino acid sequence recognized and cleaved by a Ulp1 protease. The amino acid sequence joined to the poly amino acid of interest is preferably from a SUMO (small ubiquitin-like molecule) protein. This sequence imparts favorable solubility and refolding properties to the fusion protein. A purification tag may also be incorporated into the fusion protein for ease of isolation. The Ulp1 protease used to cleave the fusion protein may be the Ulp1 protease or the active Ulp1 protease fragment, Ulp1(403-621). The Ulp1 protease rapidly and specifically cleaves the fusion proteins of the invention at the Ulp1 cleavage site. The amino acid sequence recognized by a Ulp1 protease is cleaved asymetrically to leave only an N-terminal serine joined to the poly amino acid of interest. This recombinant expression system is particularly advantageous for expression and rapid and highly specific cleavage and purification of poly amino acids that have low solubilities or are difficult to express in other systems.