A secretion signal peptide sequence (SP) in combination with a cleavage inhibition sequence (CIS) fused to a structural gene sequence in a recombinant expression system can be used to express a full length protein with an SP in a cell. Such a fusion protein may be purified to homogeneity from a membrane fraction of the cell. The SP in combination with the CIS is a protein transduction domain that exhibits superior intracellular protein transduction efficiency when the SP precedes the CIS in a N to C-terminus direction.