Protein a negative regulation of freezing tolerance

Protein phosphorylation
solely might not always be sufficient to directly alter downstream actions,
which is why dimerization in response to a post-translational modification depicts
another molecular mechanism for protein regulation. Phospho-peptide binding
proteins, such as phospho-serine / -threonine binding 14-3-3 proteins, are well
characterized (Muslin et al., 1996;
Yaffe et al., 1997; Yaffe and Elia, 2001). This
interaction can retain target proteins in a specific subcellular compartment to
either prevent or facilitate their action, as shown for the transcription
factor BRASSINAZOLE-RESISTANT 1 (BZR1). Stabilization of BIN2 under exhausted BR R1 availability leads to the phosphorylation of BZR1,
which in turn is bound by 14-3-3 protein. This leads to a maintained cytosolic
localization. Hence, BZR1 cannot be imported to the nucleus and initiate gene
expression (Gampala et al., 2007).

The direct
phosphorylation of 14-3-3 proteins however is another way of
phosphorylation-based signal transduction, as seen for cold-responsive protein kinase 1 (CRPK1)-mediated
phosphorylation of 14-3-3 proteins during cold stress (Figure 4). In
this scenario, the modified 14-3-3 protein subsequently translocates to the
nucleus, interacts with CBFs and thus promotes their degradation which causes a
negative regulation of freezing tolerance (Guo et al., 2017; Liu
et al., 2017).
Oppositely, hetero-dimerization with 14-3-3 proteins can also prevent
degradation of the target protein (Cotelle et al., 2000). This highlights that the dimerization with this
class of proteins can mediate protein regulation in response to
phosphorylation. The regulation mechanism however can be expanded to homo-/ or
hetero-dimerization processes in general, since protein phosphorylation can
confer changed binding preferences (Betts et al., 2017).

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The involvement of
different bHLH transcription factors in multiple signaling pathways was shown
to be regulated by post-translational modifications, notably by protein
phosphorylation. Hence, protein functionality was affected by either activating
or inactivating the protein in part by its stabilization or destabilization, or
by affecting the dimerization capacity of the protein. Also in the
context of Fe deprivation, protein regulation via phosphorylation plays a vital
role.

 R1BR oder
BRs