Nuclear localization sequence
The nuclear localization signal (also nuclear localization signal or nuclear localization sequence, nuclear localization signal- English, abbreviated MCB ) is one consisting of a few amino acids signal sequence that carry proteins to be introduced into the cell nucleus.
Cells of higher organisms ( eukaryotes ) have a nucleus, the nuclear membrane separating the interior of the core ( caryoplasm ) from the cytoplasm. Since all proteins are produced in the cytoplasm, proteins that are needed in the nucleus to be inserted into these. This occurs through pores in the nuclear envelope, the so-called core pores. Small proteins to about 40 kDa can passively diffuse through the pores, while larger proteins must be transported into the nucleus active. This is done with helper proteins, called importins. Importin recognize nuclear localization signals that bind to corresponding proteins and transport it through the pore to the nucleus. The energy for this process is released by the hydrolysis of GTP.
The signal itself consists of a single or two-time short sequence that contains mostly positively charged amino acids such as lysine and arginine. There are two different types, the classical NLS and the atypical NLS.
The classical nuclear localization signal sequence ( PKKKRKV ) was first described for the "large SV40 T- antigen" protein of SV40 virus. Classical NLS are further divided into one-piece and two-piece NLS. The two-part NLS ( for example, when the sequence of nucleoplasmin KR [ PAATKKAGQA ] KKKK ) consisting of two separate regions by about 10 amino acids, basic amino acids, acidic amino acids and neutral, and can improve the recording. Both types of classical NLS be by binding to Importin α and its subsequent binding to Importin β bound to the nuclear pore complex.
The atypical NLS are usually directly linked β from importin. These include, for example, the M9 domain of hnRNP A1, the repressor of transcription in yeast Matα2 ( sequence Kipik ) and U snRNPs. Ribosomal proteins have their own NLS, over which they are imported into the nucleus. The group of PY- NLS contains a characteristic proline - tyrosine sequence and binds to importin β2 ( transportin or karyopherin synonymous ).
The replication of many viruses is dependent on transport processes that are mediated by nuclear localization signals. After infection of a cell, the viral genome usually has to be transported into the nucleus. Only here you will find the host cell enzymes that can replicate and transcribe the genes of the virus. The viral DNA is therefore bound to viral proteins having a nuclear localization signal and is imported in this way into the nucleus.