Protein-histidine N-methyltransferase that specifically catalyzes 1-methylhistidine (pros-methylhistidine) methylation of target proteins (By similarity). Mediates methylation of proteins with a His-x-His (HxH) motif (where 'x' is preferably a small amino acid); 1-methylhistidine modification may affect the binding of zinc and other metals to its target proteins (By similarity). L-histidyl-[protein] + S-adenosyl-L-methionine = H(+) + N(pros)-methyl-L-histidyl-[protein] + S-adenosyl-L-homocysteine Colocalizes with membranous compartments such as the endoplasmic reticulum and mitochondria. Belongs to the METTL9 family.