Approaches to Study Phosphorylation of Intermediate Filament Proteins Using Site-Specific and Phosphorylation State-Specific Antibodies

Intermediate filaments and IF-associated proteins: from cell architecture to cell proliferation

Intermediate filaments (IFs), in coordination with microfilaments and microtubules, form the structural framework of the cytoskeleton and nucleus, thereby providing mechanical support against cellular stresses and anchoring intracellular organelles in place. The assembly and disassembly of IFs are mainly regulated by the phosphorylation of IF proteins. These phosphorylation states can be tracked using antibodies raised against phosphopeptides in the target proteins. IFs exert their functions through interactions with not only structural proteins, but also non-structural proteins involved in cell signaling, such as stress responses, apoptosis, and cell proliferation. This review highlights findings related to how IFs regulate cell division through phosphorylation cascades and how trichoplein, a centriolar protein originally identified as a keratin-associated protein, regulates the cell cycle through primary cilium formation.

Production of a site- and phosphorylation state-specific antibody

Protein phosphorylation plays important roles in various aspects of cellular events. Visualization of site-specific phosphorylation in cells is of great importance not only to analyze spatial and temporal distribution but also to investigate biological function. Now, site- and phosphorylation state-specific antibodies are widely utilized as the most powerful tools for these analyses. This protocol details a method to produce the polyclonal version of such an antibody by immunizing a synthetic phosphopeptide corresponding to a protein phosphorylated at targeted site(s). This protocol is also applicable to the production of other types of antibodies, which specifically recognize the site-specific modification, such as acetylation, methylation and proteolysis. The protocol can be completed in 2-3 months.

Regulation of mitotic function of Chk1 through phosphorylation at novel sites by cyclin-dependent kinase 1 (Cdk1)

Chk1 is phosphorylated at Ser317 and Ser345 by ATR in response to stalled replication and genotoxic stresses. This Chk1 activation is thought to play critical roles in the prevention of premature mitosis. However, the behavior of Chk1 in mitosis remains largely unknown. Here we reported that Chk1 was phosphorylated in mitosis. The reduction of this phosphorylation was observed at the metaphase-anaphase transition. Two-dimensional phosphopeptide mapping revealed that Chk1 phosphorylation sites in vivo were completely overlapped with the in vitro sites by cyclin-dependent protein kinase (Cdk) 1 or by p38 MAP kinase. Ser286 and Ser301 were identified as novel phosphorylation sites on Chk1. Treatment with Cdk inhibitor butyrolactone I induced the reduction of Chk1-S301 phosphorylation, although treatment with p38-specific inhibitor SB203580 or siRNA did not. In addition, ionizing radiation (IR) or ultraviolet (UV) light did not induce Chk1 phosphorylation at Ser317 and Ser345 in nocodazole-arrested mitotic cells. These observations imply the regulation of mitotic Chk1 function through Chk1 phosphorylation at novel sites by Cdk1.