Lactic acid bacteria (LAB) are auxotrophic for a range of amino acids and hence need to obtain them pre-formed from their environment. The LAB thus possess a wide range of proteolytic enzymes which allow them to degrade the caseins to provide the amino acids necessary to support their growth in milk (a growth substrate relatively poor in free amino acids but rich in protein). The principal proteinase in LAB is a cell wall-associated enzyme which degrades the caseins to short peptides which are further degraded by a wide range of intracellular peptidases to free amino acids. These peptidases are essential for LAB growth in milk but also contribute greatly to proteolysis in cheese during ripening. Lysis after cell death allows the peptidases to enter the cheese matrix where they act principally upon short peptides produced by proteinases during ripening.
The caseins are relatively rich in the amino acid proline. Proline has a cyclic structure that is quite different to that of all other amino acids. Hence, many "normal" peptidases are unable to degrade proline-containing peptides. Thus, LAB also have a range of proline-specific peptidases (PepX, I, Q, R, P) to allow them to utilise the caseins fully.
Although it never occurs in cheese in practice, the LAB have enzymes capable of hydrolysing every peptide bond in the casein system.
Further reading:
Upadhyay, V.K., P.L.H. McSweeney, A.A.A. Magboul and P.F. Fox (2004). Proteolysis in cheese during ripening. In Cheese: Chemistry, Physics and Microbiology, Volume 1, General Aspects, 3rd edition, P.F. Fox, P.L.H. McSweeney, T.M. Cogan and T.P. Guinee (eds), Elsevier Applied Science, Amsterdam. pp. 392-433.