The 14-3-3ζ protein binds to the cell adhesion molecule L1, promotes L1 phosphorylation by CKII and influences L1-dependent neurite outgrowth.
BACKGROUND
The cell adhesion molecule L1 is crucial for mammalian nervous system development. L1 acts as a mediator of signaling events through its intracellular domain, which comprises a putative binding site for 14-3-3 proteins. These regulators of diverse cellular processes are abundant in the brain and preferentially expressed by neurons. In this study, we investigated whether L1 interacts with 14-3-3 proteins, how this interaction is mediated, and whether 14-3-3 proteins influence the function of L1.


RESULTS
By immunoprecipitation, we demonstrated that 14-3-3 proteins are associated with L1 in mouse brain. The site of 14-3-3 interaction in the L1 intracellular domain (L1ICD), which was identified by site-directed mutagenesis and direct binding assays, is phosphorylated by casein kinase II (CKII), and CKII phosphorylation of the L1ICD enhances binding of the 14-3-3 zeta isoform (14-3-3ζ). Interestingly, in an in vitro phosphorylation assay, 14-3-3ζ promoted CKII-dependent phosphorylation of the L1ICD. Given that L1 phosphorylation by CKII has been implicated in L1-triggered axonal elongation, we investigated the influence of 14-3-3ζ on L1-dependent neurite outgrowth. We found that expression of a mutated form of 14-3-3ζ, which impairs interactions of 14-3-3ζ with its binding partners, stimulated neurite elongation from cultured rat hippocampal neurons, supporting a functional connection between L1 and 14-3-3ζ.


CONCLUSIONS
Our results suggest that 14-3-3ζ, a novel direct binding partner of the L1ICD, promotes L1 phosphorylation by CKII in the central nervous system, and regulates neurite outgrowth, an important biological process triggered by L1.
Ramser
Elisa M
EM
Zentrum für Molekulare Neurobiologie Hamburg, University of Hamburg, Hamburg, Germany.
Wolters
Gerrit
G
Dityateva
Galina
G
Dityatev
Alexander
A
Schachner
Melitta
M
Tilling
Thomas
T
D048948:14-3-3 Proteins / Q000378:metabolism*; D000818:Animals; D001665:Binding Sites; D055162:Biocatalysis; D047390:Casein Kinase II / Q000378:metabolism*; D051379:Mice; D039842:Neural Cell Adhesion Molecule L1 / Q000378:metabolism*; D016501:Neurites; D010766:Phosphorylation; D011485:Protein Binding
D048948:14-3-3 Proteins; D039842:Neural Cell Adhesion Molecule L1; D047390:Casein Kinase II