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Chapter 5.5 Protein sorting. The Structure of Proteins.

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the structure of proteins
The Structure of Proteins

Proteins consist of one or more polypeptide chains. A polypeptide consists of a series of amino acids joined ny peptide bonds. These chains are folded in a very precise fashion and if this folding is abolished than the protein generally becomes non-functional.

secondary and tertiary structure
Secondary and tertiary structure

Typically the three dimensional structure of a protein is composed of a series of fairly rigid sections strongly stabilised by hydrogen bonds (mainly alpha-helix and beta pleated sheet) and flexible zones.

The flexibility means that a molecule binding at one point on the surface of the protein will produce sympathetic changes at other points, in particular in the sites which bind the substrate and catalyse the reaction.

5.5.1 Signal hypothesis

1 signal peptide

exist in N-protein

16-26 aa,including hydrophobic center, N-protein and C-protein



  • 蛋白质的合成的命运决定于其氨基酸序列,在氨基酸序列中包含有分选信号 的,便转移到内质网膜上进行合成,这一分选信号称为信号肽。因此凡是附着到ER膜的核蛋白体,必然在它转译的肽链中包含一个氨基酸序列,作为信号指导游离的核蛋白体附着到ER膜的表面,并结合于该处;
  • 核蛋白体所转译的肽链中没有这样一个信号序列,就留在胞液中。
Cotrnaslocation :

Synthesis and transport occurred at the same time

Start transfer sequence:

lead peptide go through ER

Stop transfer sequence:

stop transfer signal
stop-transfer signal
  • Hydrophobic center of peptide,peptide have one or more stop-transfer signal,it can locate middle or end of the peptide
  • N-signal peptide is start-transfer signal,if located in the middle, consider internal signal peptide
As for mitochodria and
  • Signal petide was named leader petide

post translocation:

after protein synthesised in the cytosol,then transferd to organelle,so refered post translocation

In the process of protein transmembrane,need ATP to get rid of fold ,and some proteins to help to fold functional proteins

signal recognition particle srp
信号识别颗粒(signal-recognition particle, SRP)
  • 由6个多肽的亚单位加上小的RNA分子构成的复合物。
  • 在信号肽序列被翻译后,SRP的一端结合的新生多肽链的信号肽,另一端结合到核蛋白体并停止翻译。
  • 从SRP结合到核蛋白体直到SRP-核蛋白体复合物结合到内质网膜上SRP受体的时间内,翻译是停止的,只有在SRP-核蛋白体复合物结合到膜上的SRP受体后,翻译才重新开始。
srp srp receptor
SRP受体(SRP receptor)

是内质网膜上的一种停泊蛋白(docking protein, DP),即是一种膜的整合蛋白,暴露在胞液面,一旦它与SRP-核蛋白体复合物结合,SRP即被释放,翻译开始并通过内质网膜上的转移器,转移生长的多肽链横过膜

5.5.2 Basic pathway and type of protein sorting

post transloction:

polypeptide completed synthesis in the cytosol,and then transfer into orther organelle


  • After Protein start synthesis,peptiede transfer to RER, protein synthesis company with tansfering to RER cavity
  • 蛋白质合成起始后转移至粗面内质网中,新生肽边合成边转入粗面内质网腔中
protein sorting signal
protein sorting signal

1 Signal peptide

2 ER Retrivieval signal(KDEL)

3 Leader peptide for mitochondria

4 Nucleus location signal

  • 膜内可溶性蛋白质的转移:多肽链通过内质网膜是随着翻译同时转移到内质网腔中,这种转移方式称为协同翻译的转移。这是由于结合到内质网膜的核蛋白体利用蛋白质合成的能量,迫使其生长的多肽链穿过内质网膜上由转移器形成的通道。
  • 跨膜蛋白质的转移
Classify depend on sorting mechanism
  • Transmembrane transport
  • Vesicular transport
  • Gated transport指细胞的核、质间蛋白质运输,通过


  • Cytosol protein sorting
protein transmembrane transfer
Protein transmembrane transfer
  • Protein transmembrane transfer is more complex than soluble protein
  • stop-transfer peptide or halt transfer peptide locate in middle of peptide ,formed transmembrane protein
  • Single transmembrane: only one start transfer sequence and one stop transfer sequence
  • More times transmembrane:more start transfer sequence and more stop transfer sequence
transmembrane transport
跨膜运输(transmembrane transport)

通过结合在膜上的蛋白质转移器(protein transportor)穿过膜直接把蛋白质从细胞质运送到细胞内的不同部位。这种途径运送的蛋白质必须去折叠(unfold),这样,蛋白质更具柔韧性,可蜿蜒穿膜。如细胞质中合成后运送到线粒体、叶绿体中的蛋白质。

5.2.3 Vesicular transport Type

A Gridding protein coated vesicles网格蛋白有被小泡

B COPⅡ coated vesicles :ER—Golgi

C COPⅡ coated vesicles

retrieve escaped proteins---ER

a gridding protein coated vesicles
A Gridding protein coated vesicles
  • Transport protein from TGN golgi to cytoplasm,lysosome or vacuole
  • Golgi TGN is origin of gridding protein coated vesicles
b cop coated vesicles and regulation
B COPⅡ coated vesicles and regulation
  • ER to golgi
  • COPⅡ coated vesicles select and condense transfer material
  • COPⅡ coated vesicles can recognize cytosol signal of transmembrane ER protein

cavity of transmembrane ER protein combine with soluble protein of cavity ER


Sar protein(a small combined GTP protein) ,can act as molecular switch,regulate vesicular coat
  • Sar+GDP=inactive, Sar+GTP=active,Sar protein combine with membrane of ER,form COPⅡ coated vesicles
c cop coated vesicles
C COPⅠ coated vesicles

1 Double direction transport

Retrograde transport from Golgi to ER; anterograde transport from ER to Golgi ;

2 Main function was callback escaped protein from golgi to ER,

3 Retrieval signal(Lys-asp-glu-leu,or KDEL), golgi CGN can recognize retrieval signal,form COPⅠ coated vesicles to transfort to ER ,If protein was short of retrieval,protein cannot come back to ER

4 ER is Open prison


Many ER-resident protein bear a C-Terminal KDEL

Maily located in cis golgi network and ER-to GOLGI trasport vesicle

Chief function is bing proteinwith KDELand return them to ER

  • 蛋白二硫异构酶和分子“伴娘”
cop vesicle coat regulation
COPⅠvesicle coat regulation
  • ARF +GDP=inactive, ARF +GTP=active
  • ARF change depend on GNRP(鸟嘌呤核苷酸释放蛋白)and GAP (GTP酶激活蛋白 )
  • GNRP let ARFrelease GDPand combine with GTP,ARF becomed active,regulated COPⅠvesicle coat
  • GAP trigger ARF hydrolyze GTP into GDP, ARF becomed inactive,COPⅠ, COPⅠvesicle coat disaggregation,vesicle fused with target membrane
slide37 Transport vesicle formation

1 Selective fusion is the key factor for directional protein trasport.vesicle from ER only fused with cis CGN(高尔基体管网区)

2 Rough ER is material source ,and golgi is important distributing center

3 ER is Open prison

leader peptide
leader peptide
  • 线粒体、叶绿体中绝大多数蛋白质以及过氧化物酶体中的蛋白质也是在某种信号序列的指导下进入这些细胞器中的,这些信号称为导肽或前导肽,
  • 其基本特征为导肽或前导肽引导蛋白质跨膜过程中不仅需要ATP使多肽去折叠,而且还需要一些蛋白质的帮助(如热休克蛋白Hsp70亦称分子伴娘)使其能够正确地折叠成有功能的蛋白。
  • 由于这类蛋白质在细胞质基质中合成后再转移到这些细胞器中,因此称为后转移。
molecular chaperone
分子“伴娘”(molecular chaperone)
  • 通称为结合蛋白(bindin protein,BiP)。在细胞中这类蛋白能识别正在合成的多肽或部分折叠的多肽并与多肽的一定部位相结合,帮助这些多肽的转运、折叠或组装,但其本身并不参与最终产物的形成,只是陪伴而已,故得名
  • 例如Hsp60以上的热休克蛋白
5.5.4 Cell assemble system

Self assemble

Aid assemble

Direct assenble

Cell assemble function
  • 减少和校正蛋白合成中出现的错误
  • 大大减少所需遗传物质信息量
  • 通过装配与去装配更容易调节和控制多种生物学过程