One aspect of the present invention relates to a nucleic acid construct which includes a first nucleic acid and a second nucleic acid operatively coupled to the first nucleic acid. The first nucleic acid molecule encodes a first peptide that has the sequence of InvX or another sequence incorporating the 58 amino acid cellular import region of Mce1A and confers on Mycobacterium tuberculosis an ability to enter mammalian cells. The second nucleic acid molecule encoding a second peptide. Expression of the nucleic acid construct produces a fusion protein comprising the first peptide coupled to the second peptide. The second peptide may be a therapeutic or a diagnostic peptide. An alternative embodiment of the nucleic acid construct includes the first nucleic acid and an insertion site suitable for incorporation of the second nucleic acid molecule into this version of the construct. The present invention further relates to the fusion protein per se, screening methods for identifying fusion proteins having a protective effect against a pathogen, and therapeutic fusion proteins.