Transports DL-alanine, DL-serine and glycine (PubMed:23144255, PubMed:25535271). The preferred substrate is DL-alanine. L-serine is a low-affinity substrate (PubMed:25535271). Not stereoselective. Affinities for the D- and L- isomers of alanine and serine are in the same range. Deletion mutant is only marginally affected in medium that contains only free amino acids as a source of amino acids. Mutant shows almost no change in serine and L-threonine uptake, but shows a strong decrease in L-alanine uptake. SerP1/serP2 double mutant is completely devoid of uptake activity of either L-serine, L-threonine or L-alanine. Belongs to the amino acid-polyamine-organocation (APC) superfamily. Amino acid transporter (AAT) (TC 2.A.3.1) family.