Abstract
The divalent metal transporter 1 (DMT1) is essential for cellular iron uptake both in the intestine and in erythroid cells. We have previously shown that with iron feeding the DMT1 expressed on the brush border membrane (BBM) of the intestine undergoes endocytosis (Am. J. Physiol. 283, G965, 2002). Using the yeast two-hybrid system we have isolated a cDNA clone encoding a protein of 526-amino acid residues with a calculated molecular mass of 60 kDa, which interacts with the C-terminus of DMT1 expressed from the IRE containing mRNA (Blood ,100, 7a, 2002). The ORF of the rat protein has been fully sequenced (Genbank #AY336075) and is now designated DMT1 associated protein (DAP). DAP is ubiquitously expressed and is especially abundant in the rat intestine and colon. In rat duodenum DAP is colocalized with DMT1 in the BBM. Both by salt and pH elution DAP was demonstrated to be a peripheral membrane protein. With iron feeding both DMT1 and DAP translocate: DAP moves from the BBM to basolateral membrane (BLM) with DMT1 and some of DMT1 undergoes endocytosis and is found in cytopasmic vesicles. Immunoprecipitation and pull-down assays confirm the interaction of DAP and DMT1 in the BBM vesicles (MMBV). We have analyzed the function of DAP by exploring the role of various consensus sequences in the DAP ORF in the cellular localization of the protein. By sequence motif analysis DAP has a nuclear localization signal, Glutamic acid-rich region, Glutamine-rich region, Arginine-rich region, PKC phosphorylation sites and GOLD domain (Golgi dynamics). The region of DAP protein interacting with the COOH-terminal cytoplasmic domain of DMT1(IRE) was found to be from 171aa to 331aa which contains Glutamic acid-rich region, Glutamine-rich region and Arginine-rich region. Immunocytochemistry confirmed that DAP is localized in the nuclei and the Golgi complex of K562, MDCK, Hela, Cos1 cells, and Caco2 (where DAP is found also in BBM). GFP-fusion constructs containing the DAP nuclear localization signal (amino acids 171–277) or GOLD domain (amino acid 278–526) were transfected into COS-1 and K562 cells and specificity of intracellular localization confirmed by fluorescence confocal microscopy. DAP expression was controlled by cellular iron content: Cells which were iron depleted had increased levels of DAP protein while cells which were iron replete had decreased DAP protein. The regulation by iron was post-transcriptional as iron levels had no affect on DAP mRNA. The levels of DAP expression was also seen to affect iron uptake. Over expression of the region of DAP which binds to DMT1 by transfection of the appropriate construct into K562 cells decreased iron uptake as measured by an increase of transferrin receptor expression and decreased levels of ferritin. Elevated DAP had no affect on endogenous DMT1 expression. Conversely, when siRNAs were used to decrease DAP mRNA in K562 cells there was increased iron uptake with decreased expression of transferrin receptor and increased ferritin expression. In these experiments siRNAs reduced DAP expression by about 60%. This is the first demonstration that a protein which interacts with DMT1 can regulate the uptake of iron into the cell and suggests that DAP may act in a regulatory pathway for iron homeostasis.
Functional Identification of a Nuclear Localization Signal of MYB2 Protein in Giardia lamblia
