1 00:00:08,880 --> 00:00:21,269 Different cells secrete different types of proteins. 2 00:00:22,170 --> 00:00:26,690 Some cells, such as white blood cells, only secrete one type of protein 3 00:00:26,690 --> 00:00:29,269 and are known as unpolarized cells. 4 00:00:30,170 --> 00:00:34,469 Other cells, called polarized cells, secrete several classes of proteins 5 00:00:34,469 --> 00:00:37,390 that are each destined for delivery to a different location. 6 00:00:38,009 --> 00:00:40,149 As seen in the protein modification animation, 7 00:00:40,990 --> 00:00:43,689 some of these proteins can be destined for the endosome. 8 00:00:44,450 --> 00:00:48,509 They are targeted to the endosome by a specific mannose phosphate signal. 9 00:00:49,729 --> 00:00:52,729 Other proteins are destined for delivery outside of the cell. 10 00:00:52,729 --> 00:00:57,789 These proteins are packaged into secretory vesicles and delivered to the cell membrane. 11 00:00:58,609 --> 00:01:03,070 Some secretory vesicles congregate in the cell, awaiting an external signal. 12 00:01:03,509 --> 00:01:07,269 Once that signal is received, the vesicles rapidly fuse with the membrane 13 00:01:07,269 --> 00:01:11,189 and a large quantity of cargo proteins are released simultaneously. 14 00:01:11,890 --> 00:01:13,670 This is called regulated secretion. 15 00:01:14,390 --> 00:01:20,549 During any secretion process, the vesicle fuses with the cell membrane by a process called exocytosis. 16 00:01:21,670 --> 00:01:26,430 Alternatively, secretory vesicles are delivered to the cell membrane after they are formed 17 00:01:26,430 --> 00:01:28,909 and the protein product is immediately released. 18 00:01:29,530 --> 00:01:31,489 This is called constitutive secretion. 19 00:01:31,670 --> 00:01:35,930 The constitutive pathway is required to maintain the cell membrane 20 00:01:35,930 --> 00:01:38,670 and exists in all eukaryotic cells. 21 00:01:39,489 --> 00:01:42,390 This animation focuses on constitutive secretion. 22 00:01:43,629 --> 00:01:46,810 Protein cargo originally from the endoplasmic reticulum 23 00:01:46,810 --> 00:01:49,129 migrates through the Golgi apparatus. 24 00:01:50,189 --> 00:01:53,930 These proteins leave the trans-Golgi network in a secretory vesicle. 25 00:01:54,769 --> 00:01:58,709 Some of these proteins are not modified as they move through the Golgi apparatus. 26 00:01:59,849 --> 00:02:01,310 Unlike modified proteins, 27 00:02:01,310 --> 00:02:04,170 these proteins do not contain any signal 28 00:02:04,170 --> 00:02:08,069 that would direct their transport to a specific intracellular location, 29 00:02:08,449 --> 00:02:11,909 such as the ER, endosome, or previous Golgi cisternae. 30 00:02:12,729 --> 00:02:15,469 These proteins enter the default secretory pathway 31 00:02:15,469 --> 00:02:17,810 and are immediately secreted from the cell. 32 00:02:18,750 --> 00:02:20,990 Because there is no control over their secretion, 33 00:02:21,289 --> 00:02:23,650 this process is called constitutive secretion. 34 00:02:24,270 --> 00:02:26,830 Once the secretory vesicle reaches the cell membrane, 35 00:02:26,830 --> 00:02:29,629 it fuses immediately with the surface membrane 36 00:02:29,629 --> 00:02:34,050 and releases its cargo protein into the extracellular space. 37 00:02:34,169 --> 00:02:40,490 The secretory vesicle itself contributes new lipids and its membrane to the plasma membrane 38 00:02:40,490 --> 00:02:42,210 of the cell. 39 00:02:42,210 --> 00:02:47,569 Certain white blood cells constitutively secrete specific interleukins, which are signaling 40 00:02:47,569 --> 00:02:52,830 molecules, for the purpose of intercellular communication and play an important role in 41 00:02:52,830 --> 00:02:55,810 the function of the immune system. 42 00:02:55,810 --> 00:03:01,349 Cells such as fibroblasts constitutively secrete proteins like collagen and proteoglycans into 43 00:03:01,349 --> 00:03:06,229 the extracellular matrix and play an important role in maintaining the structural integrity 44 00:03:06,229 --> 00:03:07,430 of connective tissues.