Folding helper with both chaperone and peptidyl-prolyl cis-trans isomerase (PPIase) activities. Chaperone activity prevents aggregation of unfolded or partially folded proteins and promotes their correct folding. PPIases catalyze the cis-trans isomerization of Xaa-Pro bonds of peptides, which accelerates slow steps of protein folding and thus shortens the lifetime of intermediates. Both strategies lower the concentration of intermediates and increase the productivity and yield of the folding reaction. SlyD could be involved in Tat-dependent translocation, by binding to the Tat-type signal of folded proteins. The PPIase substrate specificity, carried out with synthetic peptides of the 'suc-Ala-Xaa-Pro-Phe-4NA' type (where Xaa is the AA tested), was found to be Phe > Ala > Leu. Required for lysis of phiX174 infected cells by stabilizing the hydrophobic viral lysis protein E and allowing it to accumulate to the levels required to exert its lytic effect. May act by a chaperone-like mechanism. Also involved in hydrogenase metallocenter assembly, probably by participating in the nickel insertion step. This function in hydrogenase biosynthesis requires chaperone activity and the presence of the metal-binding domain, but not PPIase activity. [protein]-peptidylproline (omega=180) = [protein]-peptidylproline (omega=0) PPIase activity is inhibited by binding of nickel ions to the C-terminal metal-binding region and/or the C-terminal part of the PPIase domain. Folding activity is inhibited by FK506 and by permanently unfolded proteins, irrespective of their proline content. Monomer. Binds to a broad range of unrelated Tat signal sequences. Interacts with the hydrogenase nickel incorporation protein HypB. The N-terminal region consists of two globular folded domains that contain prolyl isomerase and chaperone activities. The C-terminal region binds up to 7 nickel ions in a non-cooperative manner. Can also bind zinc with high affinity, and copper or cobalt with lower affinity. No binding detectable for ferrous, ferric, magnesium and calcium ions. Binding of nickel causes conformational rearrangements in the PPIase domain, modulating its isomerase activity. This region is also important for hydrogenase biosynthesis. Cells undergo an apoptotic-like death upon DNA damage characterized by membrane depolarization. The activity of SlyD is considerably smaller than the one found in other PPIases with the same substrate. Belongs to the FKBP-type PPIase family.