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.
Zentrum für Molekulare Neurobiologie Hamburg, University of Hamburg, Hamburg, Germany.
Elisa M
EM
Ramser
Gerrit
G
Wolters
Galina
G
Dityateva
Alexander
A
Dityatev
Melitta
M
Schachner
Thomas
T
Tilling
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