Transporter involved in exchange reactions through membranes. Has a low uniporter activity. Specifically mediates the transport of S-adenosylmethionine (SAM) and its closest analogs. Probably involved in the uptake of SAM in exchange for S-adenosylhomocysteine (SAHC), which is produced from SAM in the mitochondrial matrix and plastidial stroma by methyltransferase activities. Inhibited strongly by tannic acid, bromocresol purple, mercuric chloride, mersalyl, p-hydroxymercuribenzoate, S-adenosylhomocysteine, S-adenosylcysteine and adenosylornithine, and to a lesser extent by N-ethylmaleimide, bathophenanthroline and pyridoxal-5'-P. Detected in chloroplast envelope, but not in thylakoid membranes or in chloroplast stroma (PubMed:17098813). According to another report, localization is restricted to the mitochondria (PubMed:16950860). Expressed in seedlings, cotyledons, leaves and flowers. Lower levels of expression in stems and roots. Not detected in senescent leaves, petals and pollen grains. Severely growth-retarded phenotype and reduced chlorophyll and plastoquinone contents. Unable to germinate when homozygous. Belongs to the mitochondrial carrier (TC 2.A.29) family.