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 (PubMed:20047967). 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 (By similarity). pseudouridine(1248) in human 18S rRNA + S-adenosyl-L-methionine = H(+) + N(1)-methylpseudouridine(1248) in human 18S rRNA + S-adenosyl-L-homocysteine Homodimer. The disease is caused by variants affecting the gene represented in this entry. Belongs to the class IV-like SAM-binding methyltransferase superfamily. RNA methyltransferase NEP1 family.