A Cell Wall-associated, Receptor-like Protein Kinase

Abstract

Physical connections between higher plant cell walls and the plasma membrane have been identified visually, but the molecules involved in the contact are unknown. We describe here an Arabidopsis thaliana protein kinase, designated Wak1 for wall-associated kinase, whose predicted extracytoplasmic domain contains several epidermal growth factor repeats and identity with a viral movement protein. Wak1 fractionates with insoluble material when plant tissue is ground in a variety of buffers and detergents, suggesting a tight association with the plant extracellular matrix. Immunocytochemistry confirms that Wak1 is associated with the cell wall. Enzymatic digestion of the cell wall allows the release of Wak1 from the insoluble cell wall fraction, and protease experiments indicate that Wak1 likely has a cytoplasmic kinase domain, and the EGF containing domain is extracellular. Wak1 is found in all vegetative tissues of Arabidopsis, and has relatives in other angiosperms, but not Chlamydomonas. We suggest that Wak1 is a good candidate for a physical continuum between the cell wall and the cytoplasm, and since the kinase is cytoplasmic, it also has the potential to mediate signals to the cytoplasm from the cell wall.

Wall-Associated Kinases Are Expressed throughout Plant Development and Are Required for Cell Expansion

“…WAKs were detected on a protein gel blot using serum directed to the kinase domain conserved between the five isoforms, and the results are shown in Figure 9A. WAK serum detected a single protein band of ‫86ف‬ kD in SDS/DTT samples (lane SDS/DTT), as previously reported (He et al, 1996). After pectinase treatment, WAKs also appeared in an ‫-76ف‬kD band that accumulated with increasing time of digestion.…”

“…WAKs have an N terminus that is tightly linked to the cell wall, and a transmembrane domain separates the extracellular sequence from a carboxyl cytoplasmic serine/threonine protein kinase. WAKs can only be released from the wall by boiling in 2% SDS and DTT or by treating plant tissue with cell wall-degrading enzymes (He et al, 1996). Immunoelectronmicroscopy confirms that WAKs are associated with the cell wall.…”

“…These include cellulose synthases (Pear et al, 1996), a class of arabinogalactan proteins that are reversibly attached to the plasma membrane via a glycosyl phosphatidyl inositol anchor (Oxley and Bacic, 1999;Svetek et al, 1999), a glycolytic enzyme with a transmembrane domain (Nicol et al, 1998), and wall-associated kinases (WAKs; He et al, 1996He et al, , 1999. WAKs have the potential not only to link the ECM with the plasma membrane but also to directly 1 To whom correspondence should be addressed.…”

Two Leucine-Rich Repeat Receptor Kinases Mediate Signaling, Linking Cell Wall Biosynthesis and ACC Synthase in Arabidopsis

“…The Arabidopsis protein THE1, which belongs to the Cr RLK1L (for Catharanthus roseus protein kinase1-like) subfamily, has been hypothesized to sense cell wall integrity (Hé maty et al, 2007). A second group of RLKs, the WAKs, are tightly bound to the cell wall and likely play an important role in regulating its function (He et al, 1996;Anderson et al, 2001). Here, we describe two Arabidopsis leucine-rich repeat (LRR) RLKs in a distinct RLK clade whose disruption results in defects in cell expansion primarily in roots.…”

Genetic and physical fine mapping of the novel brown midrib gene bm6 in maize (Zea mays L.) to a 180 kb region on chromosome 2

“…In rice, a calmodulin-binding motif protein was reported to interact with wallassociated kinase (WAK)-2 like protein (Rohila et al 2006). WAKs are receptor like protein kinase, which are tightly linked with cell wall (He et al 1996), and are involved in regulation of cell elongation (Lally et al 2001) and expansion (Wagner and Kohorn 2001). This regulation requires a calcium-dependent binding protein to form the signaling complex (Decreux and Messiaen 2005).…”