Transporter for ammonium (both charged and uncharged NH3 and NH4) to use as a nitrogen source (PubMed:11069679, PubMed:11486013, PubMed:9234685, PubMed:9482721, PubMed:16477434, PubMed:18434596, PubMed:24476960, PubMed:27088325). The affinity of MEP2 is about twenty times higher than that of MEP1 (PubMed:9482721). MEP3 has the lowest affinity (PubMed:9482721). Under ammonium limitation acts as an ammonium sensor, generating a signal that leads to pseudohyphal (filamentous) growth (PubMed:9482721, PubMed:11069679, PubMed:16477434, PubMed:18434596, PubMed:32069286). Within the cytoplasmic CTD, an enhancer domain, limited to residues 428-441, upregulates substrate translocation via the MEP2 hydrophobic core, while an autoinhibitory domain, comprised within the 450-485 region and including the NPR1-target serine Ser-457, counteracts the action of the enhancer domain (PubMed:32069286). In between, a linker domain, limited to residues 442-449, appears required for optimal activity when the kinase is present but dispensable when the kinase integrity is altered (PubMed:32069286). Phosphorylated at Ser-457 by the TORC1 effector kinase NPR1 under nitrogen-limiting conditions which causes a conformational change in the C-terminal region (CTR) to form an open active conformation (PubMed:24476960, PubMed:27088325). Supplementation of nitrogen source leads to inactivation and instant Ser-457 dephosphorylation via plasma membrane PSR1 and PSR2 redundant phosphatases (PubMed:24476960). The residue Asn-4 of the protein's N-terminal tail is the only site that is glycosylated. Impairs filamentation induction. Belongs to the ammonia transporter channel (TC 1.A.11.2) family.