Model of chloroplast biogenesis. Nonphotosynthetic, undifferentiated plastids in the dark (etioplasts) import
housekeeping proteins via a translocon at the outer membrane of the chloroplast (Toc) complex consisting of
the preprotein receptors Toc132/-120 and Toc34 together with the protein-translocating channel Toc75. Light
triggers the transformation of etioplasts into chloroplasts. Developing chloroplasts import large quantities of
photosynthesis-associated proteins using a Toc complex consisting of the preprotein receptors Toc159 and Toc33
together with Toc75. The remodeling of the Toc complex during chloroplast biogenesis implicates an outer mem-
brane E3 ligase, SP1, that targets components of the protein import machinery for degradation by the 26S pro-
teasome (26SP). A, acidic; at, Arabidopis thaliana; G, GTP-binding; M, membrane insertion; RNF, RING-finger
domain; Ub, ubiquitin.
ing homology in their guanosine 5′-triphos-
phate (GTP)–binding domains, and they
extend into the cytosol. Toc75 forms the
protein-conducting channel, deeply buried
in the outer membrane. The Arabidopsis
genome revealed that a small gene family of
Toc GTPases (Toc159, -132, -120, -90, -34,
and -33) engage in separate, preprotein-
specific pathways (11, 12). Toc159 and -33
are present predominantly in the chloroplast
and mediate the import of the highly abun-
dant proteins associated with photosynthe-
sis. By contrast, Toc132/-120 and Toc34 are
present mostly in other plastid types and
are required for the import of housekeep-
Chloroplast biogenesis occurs when a
dark-grown (etiolated) plant senses the light
and consists of a series of developmental
processes called photomorphogenesis (13).
This results in extensive changes in gene
expression that dramatically increase the
components of the photosynthetic machin-
ery and enables the greening of young
plants. Greening directly reflects chloro-
plast biogenesis and leads to remodeling of
the import machinery and then of the entire
chloroplast proteome that becomes domi-
nated by highly abundant photosynthesis-
For the import machinery, the balance
is shifted from Toc120/-132 and Toc34 to
Toc159 and Toc33. Both toc33 (ppi1) (9) and
toc159 (ppi2) (10) mutants give rise to chlo-
roplast phenotypes (pale green and albino,
respectively), emphasizing the relevance of
these components and their specific role in
preprotein import. But what brings about this
switch of components in the import machin-
ery during chloroplast biogenesis?
References and Notes
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Acknowledgments: F. K. is funded by the Swiss National
Science Foundation, the National Center of Competence
in Research Plant Survival, SystemsX, and the University of
Neuchâtel. I regret not citing some work because of space