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Published 18 February 2003. doi:10.1083/jcb.200212024
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© The Rockefeller University Press, 0021-9525/2003/2/495 $5.00
The Journal of Cell Biology, Volume 160, Number 4, 495-504

Article

 Steady-state dynamics of Cajal body components in the Xenopus germinal vesicle

Korie E. Handwerger1,2, Christine Murphy1 and Joseph G. Gall1

1 Department of Embryology, Carnegie Institution of Washington, Baltimore, MD 21210
2 Department of Biology, Johns Hopkins University, Baltimore, MD 21218

Address correspondence to Joseph G. Gall, Dept. of Embryology, Carnegie Institution of Washington, 115 West University Parkway, Baltimore, MD 21210. Tel.: (410) 554-1217. Fax: (410) 243-6311. E-mail: gall{at}ciwemb.edu

Cajal bodies (CBs) are evolutionarily conserved nuclear organelles that contain many factors involved in the transcription and processing of RNA. It has been suggested that macromolecular complexes preassemble or undergo maturation within CBs before they function elsewhere in the nucleus. Most such models of CB function predict a continuous flow of molecules between CBs and the nucleoplasm, but there are few data that directly support this view. We used fluorescence recovery after photobleaching (FRAP) on isolated Xenopus oocyte nuclei to measure the steady-state exchange rate between the nucleoplasm and CBs of three fluorescently tagged molecules: U7 small nuclear RNA, coilin, and TATA-binding protein (TBP). In the nucleoplasm, the apparent diffusion coefficients for the three molecules ranged from 0.26 to 0.40 µm2 s-1. However, in CBs, fluorescence recovery was markedly slower than in the nucleoplasm, and there were at least three kinetic components. The recovery rate within CBs was independent of bleach spot diameter and could not be attributed to high CB viscosity or density. We propose that binding to other molecules and possibly assembly into larger complexes are the rate-limiting steps for FRAP of U7, coilin, and TBP inside CBs.

Key Words: coilin; diffusion; FRAP; TATA-binding protein; U7 snRNA


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