dictyNews Volume 38 Number 17

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dictyNews 
Electronic Edition 
Volume 38, number 17 
July 6, 2012 

Please submit abstracts of your papers as soon as they have been 
accepted for publication by sending them to dicty@northwestern.edu 
or by using the form at 
http://dictybase.org/db/cgi-bin/dictyBase/abstract_submit. 

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========= 
Abstracts 
========= 

 
'A Retinoblastoma Orthologue is a Major Regulator of S-phase, Mitotic,  
and Developmental Gene Expression in Dictyostelium' 

Kimchi Strasser1, Gareth Bloomfield2, Asa MacWilliams3,  
Adriano Ceccarelli4, Harry MacWilliams5, Adrian Tsang1 

1 Biology Department and Centre for Structural and Functional Genomics,  
Concordia University, Montreal, QC, Canada 
2 MRC Laboratory of Molecular Biology, Cambridge, UK and  
Wellcome Trust Sanger Institute, Hinxton, UK 
3 Siemens Corporate Technology, Munich, Germany 
4 Neuroscience Institute Cavalieri Ottolenghi, University of Torino, Italy 
5 Biozentrum der Ludwig-Maximilans-UniversitŠt, Munich, Germany 

 
PLoS ONE, in press 

The retinoblastoma tumour suppressor, Rb, has two major functions.  
First, it represses genes whose products are required for S-phase entry  
and progression thus stabilizing cells in G1. Second, Rb interacts with  
factors that induce cell-cycle exit and terminal differentiation.  
Dictyostelium lacks a G1 phase in its cell cycle but it has a retinoblastoma  
orthologue, rblA. 
Using microarray analysis and mRNA-Seq transcriptional profiling, we  
show that RblA strongly represses genes whose products are involved in  
S phase and mitosis. Both S-phase and mitotic genes are upregulated at  
a single point in late G2 and again in mid-development, near the time  
when cell cycling is reactivated. RblA also activates a set of genes unique  
to slime moulds that function in terminal differentiation. 
Like its mammalian counterpart Dictyostelium RblA plays a dual role,  
regulating cell-cycle progression and transcriptional events leading to  
terminal differentiation. In the absence of a G1 phase, however, RblA  
functions in late G2 controlling the expression of both S-phase and 
 mitotic genes. 

 
Submitted by Kimchi Strasser [kimchi@gene.concordia.ca] 
-------------------------------------------------------------------------------------- 

 
Conservation and divergence between cytoplasmic and muscle-specific  
actin capping proteins: insights from the crystal structure of cytoplasmic  
Cap32/34 from Dictyostelium discoideum. 

Eckert C, Goretzki A, Faberova M, Kollmar M. 

 
BMC Struct Biol. 2012 Jun 1;12(1):12. [Epub ahead of print] 
http://www.biomedcentral.com/1472-6807/12/12/abstract 

BACKGROUND: 
Capping protein (CP), also known as CapZ in muscle cells and Cap32/34  
in Dictyostelium discoideum, plays a major role in regulating actin filament  
dynamics. CP is a ubiquitously expressed heterodimer comprising an alpha-  
and beta-subunit. It tightly binds to the fast growing end of actin filaments,  
thereby functioning as a "cap" by blocking the addition and loss of actin  
subunits. Vertebrates contain two somatic variants of CP, one being  
primarily found at the cell periphery of non-muscle tissues while the  
other is mainly localized at the Z-discs of skeletal muscles. 

RESULTS: 
To elucidate structural and functional differences between cytoplasmic  
and sarcomercic CP variants, we have solved the atomic structure of  
Cap32/34 (32 = beta- and 34 = alpha-subunit) from the cellular slime  
mold Dictyostelium at 2.2 A resolution and compared it to that of chicken  
muscle CapZ. The two homologs display a similar overall arrangement  
including the attached alpha-subunit C-terminus (alpha-tentacle) and 
 the flexible beta-tentacle. Nevertheless, the structures exhibit marked  
 differences suggesting considerable structural flexibility within the  
 alpha-subunit. In the alpha-subunit we observed a bending motion of  
 the beta-sheet region located opposite to the position of the C-terminal  
 beta-tentacle towards the antiparallel helices that interconnect the  
 heterodimer. Recently, a two domain twisting attributed mainly to the  
 beta-subunit has been reported. At the hinge of these two domains  
 Cap32/34 contains an elongated and highly flexible loop, which has  
 been reported to be important for the interaction of cytoplasmic CP  
 with actin and might contribute to the more dynamic actin-binding of  
 cytoplasmic compared to sarcomeric CP (CapZ). 

CONCLUSIONS: 
The structure of Cap32/34 from Dictyostelium discoideum allowed  
a detailed analysis and comparison between the cytoplasmic and  
sarcomeric variants of CP. Significant structural flexibility could  
particularly be found within the alpha-subunit, a loop region in the  
beta-subunit, and the surface of the alpha-globule where the amino  
acid differences between the cytoplasmic and sarcomeric mammalian  
CP are located. Hence, the crystal structure of Cap32/34 raises the  
possibility of different binding behaviours of the CP variants toward the  
barbed end of actin filaments, a feature, which might have arisen from  
adaptation to different environments. 

 
Submitted by Martin Kollmar [mako@nmr.mpibpc.mpg.de] 
============================================================== 
[End dictyNews, volume 38, number 17]