Disease relevance of CLN3

• A cDNA library prepared from a human glioblastoma cell line has been introduced into a budding yeast strain that lacks CLN1 and CLN2 and is conditionally deficient for CLN3 function [1] .

High impact information on CLN3

• Dissociation of Whi5 is promoted by Cln3 in vivo [2].
• G1-specific transcriptional activation by Cln3/CDK initiates the budding yeast cell cycle [2].
• CLN3 functions in both daughter and mother cells of S. cerevisiae [3].
• CLN3 is not required in daughter cells, but is crucial for mother cells, in which the G1 phase is much longer in the absence of this cyclin [4].
• However, GPA1Val50 cells do not recover from division arrest in the absence of both CLN1 and CLN3, which encode G1 cyclins, indicating that the recovery-promoting activity of GPA1Val50 requires the function of G1 cyclins [5].

Biological context of CLN3

• Periodic expression of SWI4 and CLN3 may be important for cell cycle progression, as we find that these genes are both haploinsufficient and rate limiting for G1 progression [6].
• Early cell cycle box (ECB) elements, which confer M/G(1)-specific transcription, have been found in both promoters, and elimination of all ECB elements from the CLN3 promoter causes both a loss of periodicity and Cln3-deficient phenotypes, which include an extended G(1) interval and increased cell volume [7].
• Cln3-associated kinase is therefore likely to have an intrinsic in vivo substrate specificity distinct from that of Cln2-associated kinase, despite their functional redundancy [8].
• We have observed that osmotic shock causes downregulation of the kinase activity of Cln3-Cdc28 complexes [9].
• Timely transcriptional activation requires Cln3/CDK activity [10].

Anatomical context of CLN3

• Btn1p, the yeast homolog to human CLN3 that is defective in Batten disease, localizes to the vacuole [11]. This should not be confused with the yeast CLN3, which encodes the G1 type cyclin and has not been characterized to localize to the vacuole.
• The eukaryotic cell cycle is driven by a cascade of cyclins and kinase partners including the G1 cyclin Cln3p in yeast [12].
• In a cell-free system, purified Ydj1 stimulated the p34CDC28-dependent phosphorylation of the C-terminal segment of Cln3 and did not affect phosphorylation of Cln2 (as was found in vivo) [13].
• Second, cytosol prepared from cells lacking Cln3p had reduced vacuolar homotypic fusion activity in cell-free assays [14] .

Associations of CLN3 with chemical compounds

• Transcription of the CLN3 G(1) cyclin in Saccharomyces cerevisiae is positively regulated by glucose in a process that involves a set of DNA elements with the sequence AAGAAAAA (A(2)GA(5)) [15].
• The growth rate dependency of trehalose accumulation was supported by studies in cells overexpressing the G(1)-cyclin CLN3 [16].
• Replacing Ser72 with Asp, to mimic phosphorylation at an optimal Cdk-consensus site located in the first SH3 domain of Bem1p, suppressed vacuolar fragmentation in cells lacking Cln3p [17].
• Cln3 Delta, far1 Delta, and strains overexpressing Far1 do not delay budding during an ethanol glucose shift-up as wild type does [18].
• The strain containing Cln3-1 was found to be resistant to cell cycle arrest induced by exogenous phytosphingosine, indicating that Cln3 acts downstream of the sphingoid bases in this response [19].

Physical interactions of CLN3

• We conclude that AZF1 is a glucose-dependent transcription factor that interacts with the CLN3 A(2)GA(5) repeats to play a positive role in the regulation of CLN3 mRNA expression by glucose [15].
• Whi3 binds the mRNA of the G1 cyclin CLN3 to modulate cell fate in budding yeast [20].

Enzymatic interactions of CLN3

• Cln3 associates with Cdc28 to form an active kinase complex that phosphorylates Cln3 itself and a co-precipitated substrate of 45 kDa [21].

Regulatory relationships of CLN3

• First, we show that CLN3 alone is sufficient to maximally activate CLN2 transcription [22].
• Using in vivo and in vitro assays, we found that Cln3p was unable to promote vacuole fusion in the absence of Bem1p or in the presence of a nonphosphorylatable Bem1p-Ser72Ala mutant [17].
• Furthermore, activation of Cdc42p also suppressed vacuolar fragmentation in the absence of Cln3p [17].
• The G(1) cyclin Cln3 promotes cell cycle entry via the transcription factor Swi6 [23].
• Azf1 activates CLN3 transcription in Saccharomyces cerevisiae cells growing in glucose [24].

Other interactions of CLN3

• The activated mutation, CLN3-2, partially suppresses the growth defect of sgv1, suggesting that the SGV1 and CLN3 proteins may act in the same growth control pathway [5].
• Here we report our finding that the G(1) cyclin Cln3 and the S cyclin Clb5 are the key factors required for recovery from heat shock-induced G(1) arrest [25].
• Furthermore, elevated MBP1, a transcriptional regulator of cyclins, altered the transcriptional start site in CLN3 mRNA, shifting it 45 nucleotides upstream of the normal [26].
• Activation of SBF and MBF depends on the G(1) cyclin Cln3 and a largely uncharacterized protein called Bck2 [27].
• Several of these mutants identified previously known genes, including CLN3, FUS3, and FAR1 [28].
• Ydj1 is limiting for release of Cln3 and timely entry into the cell cycle [29].

Analytical, diagnostic and therapeutic context of CLN3

• As shown by indirect immunofluorescence and biochemical fractionation, Cln3p localization appears to be primarily nuclear, with the most obvious accumulation of Cln3p to the nuclei of large budded cells [30].

References