S-adenosyl-L-methionine-dependent pseudouridine N(1)-methyltransferase that methylates pseudouridine at position 1248 (Psi1248) in 18S rRNA. Involved the biosynthesis of the hypermodified N1-methyl-N3-(3-amino-3-carboxypropyl) pseudouridine (m1acp3-Psi) conserved in eukaryotic 18S rRNA. Is not able to methylate uridine at this position. Has also an essential role in 40S ribosomal subunit biogenesis independent on its methyltransferase activity, facilitating the incorporation of ribosomal protein S19 during the formation of pre-ribosomes. pseudouridine(1248) in human 18S rRNA + S-adenosyl-L-methionine = H(+) + N(1)-methylpseudouridine(1248) in human 18S rRNA + S-adenosyl-L-homocysteine Homodimer. Belongs to the class IV-like SAM-binding methyltransferase superfamily. RNA methyltransferase NEP1 family.