The genes for both of these proteins—RBCS and LHCB (also called CAB) —are present in multiple copies in the genome.

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#Unit 2. Cellular Organization

All proteins found in the nucleus, histones, transcription factors,  DNA & RNA polymerases are synthesized in the cytoplasm and imported into the nucleus through nuclear pore complexes.

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#Unit 2. Cellular Organization

These proteins contain a nuclear-localization signal (NLS) that directs their selective transport into the  nucleus.

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#Unit 2. Cellular Organization

NLSs were first discovered through the analysis of mutations of the gene for large T-antigen encoded by simian virus 40 (SV40).

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#Unit 2. Cellular Organization

The wild-type form of large T-antigen is localized to the nucleus in virus-infected cells, whereas some mutated forms of large T-antigen accumulate in the cytoplasm.

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#Unit 2. Cellular Organization

The mutations responsible for this altered cellular localization occur within a specific seven-residue sequence rich in basic amino acids near the C-terminus of the protein: Pro-Lys-Lys-Lys-Arg-Lys-Val.

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#Unit 2. Cellular Organization

Nuclear transport receptor

A general class of proteins that facilitate protein transport into or out of the nucleus by binding to a nuclear import sequence or a nuclear export sequence