Enzyme of the methionine cycle that catalyzes the irreversible cleavage of the glycosidic bond in 5'-methylthioadenosine (MTA) to adenine and 5'-methylthioribose (PubMed:19249293, PubMed:18342331). Contributes to the maintenance of AdoMet homeostasis and is required to sustain high rates of ethylene synthesis. Inactive towards S-adenosylhomocysteine (SAH/AdoHcy) (PubMed:18342331, PubMed:19249293). H2O + S-methyl-5'-thioadenosine = 5-(methylsulfanyl)-D-ribose + adenine Inhibited by CBL3 in a calcium-dependent manner (PubMed:18945934). Inhibited by 5'-methylthiotubercidin (MTT) and by formycin A (FMA) (PubMed:18342331). kcat is 18.7 sec(-1). Optimum pH is 8.0. Amino-acid biosynthesis; L-methionine biosynthesis via salvage pathway; S-methyl-5-thio-alpha-D-ribose 1-phosphate from S-methyl-5'-thioadenosine (hydrolase route): step 1/2. Homodimer (PubMed:19249293, PubMed:16909418). Interacts with CBL3 in a calcium-dependent manner. Expressed in roots, leaves, stems, cauline leaves and flowers. No visible phenotype under normal growth condition. Not able to grow with methylthioadenosine (MTA) as unique source of sulfur. Belongs to the PNP/UDP phosphorylase family. MtnN subfamily.