Ideal immunogenicity in antigens is a prerequisite to eliciting a sufficiently strong immune and memory response via either DNA or protein vaccines. To improve immunogenicity, efforts have focused on high-level expression of target proteins and on maintaining their natural conformations. In the present work, two trimer motifs (MTQ and MTI) were designed and introduced into a plasmid vector with the tissue plasminogen activator signal peptide (tPA-SP). Next, we examined the efficacy and the efficiency of the two motifs as well as the introduction of tPA-SP and its mutant forms, 22P/A and 22P/G, in facilitating the secretory expression of trimeric proteins in mammalian cells. We found that both trimer motifs could produce the target protein in a trimeric form at a high level. Introduction of tPA-SP 22P/A markedly increased the secretory expression level. The combination of the trimer motif, MTQ, and the signal peptide, 22P/A, may serve as a universal mammalian vector for producing trimeric proteins in vaccine development.