Multisite phosphorylation of doublecortin by cyclin-dependent kinase 5

Area of Science:

• Neuroscience
• Molecular Biology
• Biochemistry

Background:

• Doublecortin (DCX) is crucial for neural migration and cortical development.
• Mutations in DCX and defects in cyclin-dependent kinase 5 (cdk5) disrupt neural development.
• DCX is a known substrate for cdk5, with Ser-297 previously identified as a major in vitro phosphorylation site.

Purpose of the Study:

• To comprehensively identify and characterize cdk5 phosphorylation sites on Doublecortin (DCX) using advanced mass spectrometry.
• To determine the major and minor sites of cdk5 phosphorylation on DCX in vitro and in vivo.
• To investigate the functional significance of identified phosphorylation sites through mutagenesis studies.

Main Methods:

• Utilized a multi-stage mass spectrometry (MS) strategy, including HPLC and MALDI-MS.
• Employed alkaline phosphatase treatment to aid phosphopeptide identification.
• Performed tandem MS/MS for detailed analysis of phosphopeptides.
• Conducted site-directed mutagenesis to validate MS findings.

Main Results:

• Identified seven distinct cdk5 phosphorylation sites on DCX.
• Determined Ser-28 (N-terminus) and Ser-339 (C-terminus) as major phosphorylation sites.
• Found five additional sites: Ser-287, Thr-289, Ser-297, Thr-326, and Ser-332.
• Demonstrated that mutations at Ser-332 and Ser-339 significantly reduced overall phosphorylation, suggesting site interaction.
• Showed that truncating the DCX tail markedly decreased cdk5 phosphorylation.

Conclusions:

• DCX undergoes complex, multi-site phosphorylation by cdk5, challenging the notion of Ser-297 as the sole major site.
• The N-terminus (Ser-28) and C-terminus (Ser-339) are key phosphorylation sites.
• Phosphorylation of DCX by cdk5 is regulated by interactions between multiple sites, particularly in the C-terminal tail.
• Highlights the necessity of sophisticated MS strategies for accurate identification of phosphorylation sites in biological regulation.