Catalyzes the sodium-dependent uptake of extracellular L-proline (PubMed:3512540, PubMed:1567896, PubMed:9693004). This protein is also capable of using lithium as the transport cation (PubMed:1567896, PubMed:9693004). Also catalyzes the uptake of propionate (PubMed:17088549). L-proline(in) + Na(+)(in) = L-proline(out) + Na(+)(out) Activity is stimulated by phosphatidylethanolamine and phosphatidylglycerol, but not by phosphatidylcholine and cardiolipin (PubMed:3512540). Proline uptake is inhibited by the sulfhydryl reagent N-ethylmaleimide (NEM) (PubMed:3053687, PubMed:1567896). Proline, in the presence of Na(+) or Li(+), protects the carrier functions from NEM-inactivation (PubMed:1567896). Optimum pH is 7.0. Has been isolated from inner membrane preparations as a homodimer. Up-regulated by propionate. Mutant is defective in proline transport (PubMed:3007935). Shows reduced growth rate when grown on propionate as sole carbon source (PubMed:17088549). Belongs to the sodium:solute symporter (SSF) (TC 2.A.21) family.