Iron-responsive element-binding protein. Phosphorylation by protein kinase C

Area of Science:

• Biochemistry
• Molecular Biology
• Cell Biology

Background:

• The iron-responsive element-binding protein (IRE-BP) is crucial for maintaining cellular iron homeostasis by regulating the synthesis of transferrin receptor and ferritin.
• Factors beyond iron that modulate IRE-BP activity are not well understood, representing a gap in knowledge regarding cellular iron utilization control.

Purpose of the Study:

• To investigate the role of protein kinase C (PKC) in modulating the activity of the iron-responsive element-binding protein (IRE-BP).
• To explore the potential mechanism of IRE-BP phosphoregulation by PKC in cellular iron metabolism.

Main Methods:

• Purified rat liver IRE-BP was subjected to phosphorylation by PKC, with analysis of modified amino acids and phosphopeptide mapping.
• Phosphorylation sites (Ser 138 and Ser 711) were identified using synthetic peptides.
• HL 60 cells and rat fibroblasts were treated with phorbol 12-myristate 13-acetate (PMA) to stimulate PKC, followed by assessment of IRE-BP phosphorylation and RNA binding activity.

Main Results:

• PKC phosphorylated purified rat liver IRE-BP at serine residues, with a Km of 0.4 microM.
• PKC phosphorylated synthetic peptides containing Ser 138 and Ser 711.
• PMA treatment in HL 60 cells increased IRE-BP phosphorylation and high-affinity IRE RNA binding activity up to 3-fold within 90 minutes.
• PMA did not alter [35S]Met incorporation into IRE-BP, indicating no effect on protein synthesis.

Conclusions:

• Phosphorylation by PKC represents a novel mechanism for regulating IRE-BP function.
• Hormones and growth factors may control cellular iron utilization by specifically phosphoregulating IRE-BP.
• This study elucidates a new pathway for managing cellular iron levels through post-translational modification of IRE-BP.