Distinguish between preproproteins and proproteins

Preproproteins and proproteins are both forms of precursor proteins synthesized in the cell before they are processed into their mature, functional forms. Here's how they differ:

1. Preproproteins:

   • Preproproteins are the earliest forms of precursor proteins synthesized by ribosomes in the cell.
   • They contain an N-terminal signal sequence or signal peptide, which directs the protein to the endoplasmic reticulum (ER) for translocation into the ER lumen or ER membrane.
   • The signal sequence is typically cleaved off during translocation, resulting in the removal of a portion of the protein.
   • After translocation, preproproteins are referred to as proproteins.

2. Proproteins:

   • Proproteins are intermediate forms of precursor proteins that have been translocated into the ER lumen or ER membrane.
   • They retain part of the signal sequence, known as the signal anchor sequence, which remains attached to the protein.
   • Proproteins may undergo further processing within the ER, Golgi apparatus, or other cellular compartments to attain their mature, functional forms.
   • Processing of proproteins may involve cleavage of additional signal sequences, removal of specific peptide segments, or post-translational modifications such as glycosylation or phosphorylation.

In summary, preproproteins are the initial precursors synthesized by ribosomes and contain a signal sequence that targets them to the ER. Upon translocation into the ER, preproproteins are cleaved to become proproteins, which retain a portion of the signal sequence. Proproteins then undergo further processing and modifications to attain their mature, functional forms.