B10: Initial steps of thylakoid membrane formation and the role of endomembrane system-mediated protein targeting into plastids

Early seedling development comprises the differentiation of chloroplasts from their progenitors, the proplastids. These precursors are surrounded by a double membrane and contain only rudimentary vesicles or lamellae. During the transition of proplastids into chloroplasts the intricate network of the thylakoid membrane system progressively develops coupled with a massive flow of lipids, pigments and proteins from the inner and outer membrane into the interior of the organelle. There is hardly any research on how thylakoid membranes originate in higher plants. Recent investigations in cyanaobacteria suggest that initial steps of thylakoid biogenesis are located in a specialized membrane subfraction, the so-called biosynthetic centers, which are in contact to both the thylakoids and the plasma membrane. Microscopic analyses showed that in proplastids of higher plants the inner envelope is involved in these initial steps, visible as invaginations or vesicle formation at this membrane layer. The functional consequence of these structures has not been investigated so far. In this proposal we aim to investigate the dynamics of thylakoid membrane formation, thereby we will focus on fundamental events ensuring that this sophisticated membrane system and its individual identity is established.

Membrane biogenesis in plastids requires the biosynthesis of lipid precursors and further non-polar metabolites provided by the endoplasmic reticulum (ER) and the import of numerous structural and auxiliary proteins. Although, the majority of nuclear-encoded chloroplast proteins are imported by the translocons in the outer and inner envelope, non-canonical pathways exist. Surprisingly, some chloroplast proteins are transported via the endomembrane system. The relevance of this non-canonical import route for chloroplast development is unresolved because until now only non-essential proteins are known, which are clients of this pathway. The endomembrane system-mediated transport process is suggested to follow the classical secretory pathway, however, the import into chloroplasts and cargo sorting as well as the involved protein components are completely unknown.

In a forward genetic approach we have identified the nuclear gene HCF222, whose product is targeted to plastids via the endomembrane system-mediated import pathway. HCF222 is indispensable for establishing a functional thylakoid membrane system in chloroplasts, thus this factor for the first time offers an entry point for the genetic examination of this transport route.

The present project is centred on two complementary goals. By exploiting the thylakoid biogenesis factor HCF222 we intend to analyse the endomembrane system-mediated import pathway into chloroplasts. We aim to (i) identify specific components involved in this transport process and to (ii) investigate the involved transport mechanisms, especially those at the chloroplast surface.

The second subproject uses reverse genetics to (i) identify novel protein factors required for the initial steps of thylakoid formation and to (ii) investigate structural modifications involved in these processes.

Project leaders:
PD Dr. Karin Meierhoff, undefined email, and Prof. Dr. Peter Westhoff, undefined email,
                          undefinedInstitute of Developmental and Molecular Biology of Plants

Researcher: Jasmin Mehsing, undefined email

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