Disordered Motifs and Domains in Cell Control
Sunday Speaker Abstracts
Intrinsic Disorder in Membrane Proteins
Birthe B. Kragelund
, Gitte W. Haxholm, Louise F. Nikolajsen, Ruth Hendus-Altenburger.
University of Copenhagen, Copenhagen N, Denmark.
Intrinsically disordered regions (IDRs) in membrane proteins play central roles in cellular
signaling processes and like their structured protein counterparts, they engage in interaction
networks of regulatory nature. Intracellular domains of many membrane proteins contain large
IDRs of importance for function and with numerous predicted as well as confirmed
phosphorylation sites. Due to their lack of globular structure insight into their structure-function
relations have been crucially lacking. Using NMR spectroscopy, biophysics and cell-biology we
have deciphered regulatory roles of intrinsic disorder in cytokine receptors and in ion
transporters with direct links to phosphorylations. The interplay of intrinsic disorder and
phosphorylation in these proteins highlights specific space and temporal effects in scaffolding
including interplay with some of the major signaling pathways such as JAK2/STAT and MAPK-
signaling.
Multisite Phosphorylation Networks in Cdk1-dependent Cell Cycle Regulation
Rainis Venta
1
, Andreas Doncic
2
, Ervin Valk
1
, Mardo Kõivomägi
1
, Jan Skotheim
2
,
Mart Loog
1
.
1
Institute of Technology, University of Tartu, Tartu, Estonia,
2
Stanford University, Stanford, CA,
USA.
Multisite phosphorylation of proteins is a powerful signal processing mechanism whose diverse
possibilities are not well understood. In this process, multiple phosphates are added in either a
random or defined order to kinase substrates. When a crucial set of key sites becomes
sufficiently phosphorylated, a downstream signaling switch is triggered. Multisite
phosphorylation plays a pivotal role in processing CDK (cyclin-dependent kinase) signals to
ensure temporal regulation of cell cycle events. The key factor that controls this process is the
phospho-adaptor Cks1. It binds to phosphorylated threonines and enhances CDK-dependent
phosphorylation of neighboring sites. This event occurs several times to process the
phosphorylation of the entire network of sites. As the phosphorylation sites are located in
disordered segments of the targets, or in disordered proteins, the cyclin-CDK-Cks1 complex acts
as a catalytic scaffold whose rate of multisite phosphorylation is determined by how well the
fixed spatial orientation of the docking pockets on the scaffold fits with the linear pattern of
phosphorylation sites and docking sites in the substrates. We demonstrate this phenomenon on
Sic1, a G1/S inhibitor of Cdk1 in budding yeast. The phosphorylation events in Sic1 lead to the
generation of phosphodegron motifs that are recognized by the ubiquitination machinery and
thereby mark Sic1 for destruction. We show that the network of sites in Sic1 is processively
phosphorylated by S-phase cyclin-Cdk1-Cks1. The processivity is modulated by
phosphorylation/dephosphorylation of a priming site and a diversional site by two kinases and a
phosphatase of stress pathways. Both the priming site and the diversional site compete for
binding to Cks1. This mechanism demonstrates how external signals can be integrated into the
Cdk1 control system via multi-branched signal-processing modules based on multisite
phosphorylation networks. Such transistor-like modules are possibly ubiquitous and could
regulate many cellular events.
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