Читать книгу An Introduction to Molecular Biotechnology - Группа авторов - Страница 47

Michael Wink

Heidelberg University, Institute of Pharmacy and Molecular Biotechnology (IPMB), Im Neuenheimer Feld 329,, 69120 Heidelberg, Germany

The cellular compartments were introduced in Chapter 3. All compartments are enclosed by a biomembrane and contain a multitude of proteins. In many cases, the separation of proteins in a cell is compartment specific, meaning that every compartment harbors its own set of proteins. Every animal cell contains about 10¹⁰ single protein molecules, whose synthesis begins on the ribosomes in the cytoplasm. Every protein must finally arrive in the part of the cell where it is to be functional. One of the central questions in molecular biology concerns the mechanism of protein sorting. The understanding of this issue is important for biotechnology, especially when it comes to direct recombinant proteins into the correct compartments.

Three important pathways of protein sorting (Figure 5.1) are known:

 Gated transport: Transport of proteins and RNA via the nuclear pore complex(NPC) into and out of the cell nucleus. The nuclear pores exhibit selective channeling, allowing entry only for certain macromolecules. The export out of the nucleus also proceeds selectively via nuclear pores.

 Protein translocation: Uptake of a protein produced in the cytosol by an organelle via specific protein translocators. This is the pathway for proteins taken up by the mitochondria, plastids, and peroxisomes.

 Vesicular transport: Proteins secreted in the endoplasmic reticulum(ER) undergo a series of posttranslational modifications in the ER and in the Golgi apparatus. The finished proteins are packed into vesicles and sent to the lysosomes, endosomes, or cytoplasmic membrane. There the vesicle fuses with the membranes of the organelles or the cell, and the content of the vesicle is released through exocytosis.

Figure 5.1 Schematic overview of protein transport inside a cell.

The selectivity of protein transport is based on recognition signals that proteins must carry. If a protein does not have a signal, it remains in the cytoplasm. All other proteins contain address labels that determine the designated location. They are either coherent signal sequences, with 15–60 amino acids, or signal patches, which are only recognizable in a three‐dimensional state and are made up of signal sequences from many protein domains. The signal sequences are very conserved in their structure. Important examples are shown in Table 5.1. Signal sequences are usually found on the N‐ or C‐terminal of a protein. They are usually removed by signal peptidases as soon as a protein has reached its destination.

Table 5.1 Examples of typical recognition sequences.

Targeted compartment	Sequence
Nuclear import	‐Pro‐Pro‐Lys‐Lys‐Lys‐Arg‐Lys‐Val‐
Nuclear export	‐Met‐Glu‐Glu‐Leu‐Ser‐Gln‐Ala‐Leu‐Ala‐Ser‐Ser‐Phe‐ ‐
Mitochondria	H3N+‐Met‐Leu‐Ser‐Leu‐Arg‐Gln‐Ser‐Ile‐Arg‐Phe‐Phe‐Lys‐Pro‐Ala‐Thr‐
	Arg‐Thr‐Leu‐Cys‐Ser‐Ser‐Arg‐Tyr‐Leu‐Leu‐
Plastids	H3N+‐Met‐Val‐Ala‐Met‐Ala‐Met‐Ala‐Ser‐Leu‐Gln‐Ser‐Ser‐Met‐Ser‐Ser‐
	Leu‐Ser‐Leu‐Ser‐Ser‐Asn‐Ser‐Phe‐Leu‐Gly‐Gln‐Pro‐Leu‐Ser‐Pro‐Ile‐Thr‐ Leu‐Ser‐Pro‐Phe‐Leu‐Gln‐Gly‐
Peroxisomes	‐Ser‐Lys‐Leu‐COO–
ER import	H3N+‐Met‐Met‐Ser‐Phe‐Val‐Ser‐Leu‐Leu‐Leu‐Val‐Gly‐Ile‐Leu‐Phe‐Trp‐
	Ala‐Thr‐Glu‐Ala‐Glu‐Gln‐Leu‐Thr‐Lys‐Cys‐Glu‐Val‐Phe‐Gln‐
ER retention	‐Lys‐Asp‐Glu‐Leu‐COO–‐

Amino acids printed in bold are especially important for the signal sequence.