An in vitro method of enhancing the level of expression of a target protein of interest produced by a host cell comprising transfecting the host cell with an expression vector comprising a structural gene encoding a fusion protein comprising two polypeptide domains: a protein expression enhancing polypeptide (PEEP) domain linked to a target protein binding domain, wherein said PEEP domain is: (a) an isolated J domain of a J protein comprising a polypeptide domain from a J protein characterized by four α-helices (I, II, III and IV) and having the tripeptide sequence of histidine, proline, and aspartic acid sequence motif (HPD motif) between helix II and helix III; or (b) a J domain analog polypeptide comprising the amino acid sequence of formula I: X1-X2-X3-X4-X5-X6-X7-X8-X9 (SEQ ID NO:47), wherein: X1 is isoleucine, leucine, valine, alanine, or methionine; X2 and X3 are each independently any amino acid with the proviso that one or both are K or R; X4 is any amino acid or X4 may be absent when X1 through X3 are present and X5 through X9 are present; X5 is tyrosine, tryptophan, or phenylalanine; X6 and X7 are each independently any amino acid with the proviso that one or both are lysine or arginine; or either one of X6 and X7 may be absent when the other is K or R and when X1 through X5 are present and X8 and X9 are present; and X8 and X9 are any amino acid with the proviso that one or both are leucine or alanine; or one of X8 and X9 may be absent when the other is L or A and when X1 through X7 are present; and wherein said TPB domain comprises a polypeptide capable of binding said target protein of interest; wherein the TPB domain is an antibody or an antigen-binding fragment thereof, an immunoglobulin-specific binding protein, a ligand binding domain of a receptor protein, a protein ligand of a target protein of interest, or a PDZ domain; wherein co-expression of said fusion protein with said target protein of interest in said host cell results in an enhanced level of expression of said target protein of interest compared to the level of target protein expressed by said host cell in the absence of said fusion protein co-expression. (a) an isolated J domain of a J protein comprising a polypeptide domain from a J protein characterized by four α-helices (I, II, III and IV) and having the tripeptide sequence of histidine, proline, and aspartic acid sequence motif (HPD motif) between helix II and helix III; or (b) a J domain analog polypeptide comprising the amino acid sequence of formula I: X1-X2-X3-X4-X5-X6-X7-X8-X9 (SEQ ID NO:47), wherein: X1 is isoleucine, leucine, valine, alanine, or methionine; X2 and X3 are each independently any amino acid with the proviso that one or both are K or R; X4 is any amino acid or X4 may be absent when X1 through X3 are present and X5 through X9 are present; X5 is tyrosine, tryptophan, or phenylalanine; X6 and X7 are each independently any amino acid with the proviso that one or both are lysine or arginine; or either one of X6 and X7 may be absent when the other is K or R and when X1 through X5 are present and X8 and X9 are present; and X8 and X9 are any amino acid with the proviso that one or both are leucine or alanine; or one of X8 and X9 may be absent when the other is L or A and when X1 through X7 are present; and wherein said TPB domain comprises a polypeptide capable of binding said target protein of interest; wherein the TPB domain is an antibody or an antigen-binding fragment thereof, an immunoglobulin-specific binding protein, a ligand binding domain of a receptor protein, a protein ligand of a target protein of interest, or a PDZ domain; wherein co-expression of said fusion protein with said target protein of interest in said host cell results in an enhanced level of expression of said target protein of interest compared to the level of target protein expressed by said host cell in the absence of said fusion protein co-expression. X1 is isoleucine, leucine, valine, alanine, or methionine; X2 and X3 are each independently any amino acid with the proviso that one or both are K or R; X4 is any amino acid or X4 may be absent when X1 through X3 are present and X5 through X9 are present; X5 is tyrosine, tryptophan, or phenylalanine; X6 and X7 are each independently any amino acid with the proviso that one or both are lysine or arginine; or either one of X6 and X7 may be absent when the other is K or R and when X1 through X5 are present and X8 and X9 are present; and X8 and X9 are any amino acid with the proviso that one or both are leucine or alanine; or one of X8 and X9 may be absent when the other is L or A and when X1 through X7 are present; and