T he glucose transporter 1 (GLUT1) protein is underexpressed in human glioblastoma multiforme and is overexpressed in human cerebral hemangioblastoma. To gain in-sight into possible posttranscriptional mechanisms regulating the expression of the GLUT1 protein in human brain tumors, cytosolic proteins were prepared from these two tumors and used in RNase T1 protection assays that employed [32P]human GLUT1 synthetic RNA prepared from transcription plasmids. Gel shift mobility assays and ultra-violet light cross-linking studies demonstrated the formation of specific RNA/protein complexes that migrated with a mol mass of 120, 44, and 41 kD. RNase T1 mapping and oligodeoxynucleotide competition studies showed that the 120 kD complex was comprised of an RNA fragment that localized to nucleotides 2186-2203 of the GLUT1 mRNA. The 44 kD complex contained an adenosine-uridine-rich RNA fragment that localized to nucleotides 1885-1906 of the human GLUT1 mRNA, and the formation of this complex was inhibited by synthetic RNA enriched in adenosine-uridine sequences. The 44 kD complex was selectively downregulated in hemangioblastoma as compared to glioblastoma multiforme. These studies demonstrate that human brain tumors have differential regulation of cytosolic proteins that specifically interact with two different domains in the 3'-untranslated region of the GLUT1 mRNA, which may serve to mediate the posttranscriptional regulation of GLUT1 gene expression in these tumors.